Controlling light sources of a directional backlight

ABSTRACT

Disclosed is an imaging directional backlight including an array of light sources, and a control system arranged to provide variable distribution of luminous fluxes, scaled inversely by the width associated with the respective light sources in the lateral direction, across the array of light sources. The luminous intensity distribution of output optical windows may be controlled to provide desirable luminance distributions in the window plane of an autostereoscopic display, a directional display operating in wide angle 2D mode, privacy mode and low power consumption mode. Image quality may be improved and power consumption reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent App. No. 61/649,050,entitled “Control system for a directional light source”, filed 18 May2012 (RealD Ref: 319000), and to U.S. Patent App. No. 61/791,928,entitled “Illumination apparatus and control system for a directionaldisplay device”, filed 15 Mar. 2013 (RealD ref: 348000), the entiretiesof which are herein incorporated by reference.

TECHNICAL FIELD

This disclosure generally relates to illumination of spatial lightmodulators, and more specifically relates to directional backlights forproviding large area illumination from localized light sources for usein 2D, 3D, and/or autostereoscopic display devices.

BACKGROUND

Spatially multiplexed autostereoscopic display devices typically align aparallax component such as a lenticular screen or parallax barrier withan array of images arranged as at least first and second sets of pixelson a spatial light modulator, for example an LCD. The parallax componentdirects light from each of the sets of pixels into different respectivedirections to provide first and second viewing windows in front of thedisplay. An observer with an eye placed in the first viewing window cansee a first image with light from the first set of pixels; and with aneye placed in the second viewing window can see a second image, withlight from the second set of pixels.

Such display devices have reduced spatial resolution compared to thenative resolution of the spatial light modulator and further, thestructure of the viewing windows is determined by the pixel apertureshape and parallax component imaging function. Gaps between the pixels,for example for electrodes, typically produce non-uniform viewingwindows. Undesirably such displays exhibit image flicker as an observermoves laterally with respect to the display and so limit the viewingfreedom of the display. Such flicker can be reduced by defocusing theoptical elements; however such defocusing results in increased levels ofimage cross talk and increases visual strain for an observer. Suchflicker can be reduced by adjusting the shape of the pixel aperture,however such changes can reduce display brightness and can includeaddressing electronics in the spatial light modulator.

BRIEF SUMMARY

According to a first aspect of the present disclosure, there is provideda method of controlling an array of light sources of a directionalbacklight. The directional backlight may include a waveguide having aninput end and the array of light sources may be disposed at differentinput positions in a lateral direction across the input end of thewaveguide. The waveguide may have first and second opposed guidesurfaces for guiding light along the waveguide. The waveguide may bearranged to direct input light from the light sources, as output lightthrough the first guide surface into optical windows in outputdirections distributed in a lateral direction to the normal to the firstguide surface that may be primarily dependent on the input positions.The method may selectively operate the light sources to direct lightinto varying optical windows corresponding to the output directions, andthe light sources may be controlled to output light with luminousfluxes, scaled inversely by the width associated with the respectivelight sources in the lateral direction, that vary across the array oflight sources.

In some embodiments, the variation of display luminance with viewingangle may be modified to achieve a backlight that has Lambertiancharacteristics from waveguides that exhibit non-Lambertian opticaloutput characteristics when illuminated by light source arrays withuniform luminous fluxes across the array of light sources.Advantageously, the backlight may appear to have a visual appearancesimilar to paper and may be comfortable to view as each eye of anobserver may perceive an illumination structure with the same perceivedimage brightness.

In other embodiments, the luminance of the backlight may be arranged tofall for off axis viewing positions at a faster rate than for aLambertian characteristic. By way of comparison, such a backlight mayachieve substantially lower power consumption in comparison toLambertian output backlights.

In observer tracked embodiments, the backlight may achieve a Lambertianillumination appearance for a given viewing position, while the changein intensity with viewing position may vary in a non-Lambertian manner.Advantageously, the image may have high comfort to view, while thebacklight power consumption may be reduced in comparison to Lambertiandisplays.

In further embodiments, the backlight luminance may be arranged to varyacross a viewing window; the cross talk of an autostereoscopic image maybe reduced while achieving acceptable levels of image flicker for amoving observer.

According to another aspect of the present disclosure, there is provideda directional display apparatus in which a control system is arranged toimplement a similar method.

According to another aspect of the present disclosure, there is provideda method of controlling an array of light sources of a directionalbacklight. The directional backlight may include a waveguide having aninput end and the array of light sources disposed at different inputpositions in a lateral direction across the input end of the waveguide.The waveguide may further include first and second opposed guidesurfaces for guiding light along the waveguide, and a reflective endfacing the input end for reflecting input light from the light sourcesback through the waveguide. The waveguide may be arranged to directinput light from the light sources, after reflection from the reflectiveend, as output light through the first guide surface into opticalwindows in output directions distributed in a lateral direction to thenormal to the first guide surface that may be primarily dependent on theinput positions. The method may include supplying drive signals to thelight sources that selectively operate the light sources to direct lightinto varying optical windows corresponding to the output directions andsensing light incident on the input end from the light sources afterreflection from the reflective end. The drive signals may be calibratedin response to the sensed light incident on the input end.

The scaled luminous flux of a light source array may vary spatially dueto non-uniformities in luminance and chromaticity between respectivelight source of the array. The present embodiments may advantageouslyachieve calibration of variation between fluxes of respective lightsources so that an optical output may be provided that achieves controlof light source scaled luminous flux that may vary in a uniform mannerfor an observer. Further, the scaled luminous flux may vary with time,for example due to ageing effects of the light source. The presentembodiments may achieve in-field correction of light source ageingeffects, advantageously providing extended device lifetime anduniformity of output. A small number of detectors may be used to monitorthe whole array during a calibration step, reducing cost.

Sensing the light incident on the input end may use sensor elementsarranged at a region of the input end outside the array of light sourcesin the lateral direction. In another example, sensing the light incidenton the input end may use sensor elements arranged at regions of theinput end outside the array of light sources in the lateral direction onboth sides of the array of light sources. In yet another example,sensing the light incident on the input end may use light sources of thearray that are not concurrently operated. The levels of the drivesignals may be calibrated such that the light sources output light withluminous fluxes that have a predetermined distribution across the arrayof light sources

According to another aspect of the present disclosure, there is provideda directional backlight apparatus in which a control system is arrangedto implement a similar method.

Embodiments herein may provide an autostereoscopic display with largearea and thin structure. Further, as will be described, the opticalvalves of the present disclosure may achieve thin optical componentswith large back working distances. Such components can be used indirectional backlights, to provide directional displays includingautostereoscopic displays. Further, embodiments may provide a controlledilluminator for the purposes of an efficient autostereoscopic andefficient 2D image display.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiment may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audiovisual systems and electrical and/or opticaldevices. Aspects of the present disclosure may be used with practicallyany apparatus related to optical and electrical devices, opticalsystems, presentation systems or any apparatus that may contain any typeof optical system. Accordingly, embodiments of the present disclosuremay be employed in optical systems, devices used in visual and/oroptical presentations, visual peripherals and so on and in a number ofcomputing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

Directional backlights offer control over the illumination emanatingfrom substantially the entire output surface controlled typicallythrough modulation of independent LED light sources arranged at theinput aperture side of an optical waveguide. Controlling the emittedlight directional distribution can achieve single person viewing for asecurity function, where the display can only be seen by a single viewerfrom a limited range of angles; high electrical efficiency, whereillumination is only provided over a small angular directionaldistribution; alternating left and right eye viewing for time sequentialstereoscopic and autostereoscopic display; and low cost.

The various aspects of the present invention and the various featuresthereof may be applied together in any combination.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating a front view of lightpropagation in one embodiment of a directional display device, inaccordance with the present disclosure;

FIG. 1B is a schematic diagram illustrating a side view of lightpropagation in one embodiment of the directional display device of FIG.1A, in accordance with the present disclosure;

FIG. 2A is a schematic diagram illustrating in a top view of lightpropagation in another embodiment of a directional display device, inaccordance with the present disclosure;

FIG. 2B is a schematic diagram illustrating light propagation in a frontview of the directional display device of FIG. 2A, in accordance withthe present disclosure;

FIG. 2C is a schematic diagram illustrating light propagation in a sideview of the directional display device of FIG. 2A, in accordance withthe present disclosure;

FIG. 3 is a schematic diagram illustrating in a side view of adirectional display device, in accordance with the present disclosure;

FIG. 4A is schematic diagram illustrating in a front view, generation ofa viewing window in a directional display device and including curvedlight extraction features, in accordance with the present disclosure;

FIG. 4B is a schematic diagram illustrating in a front view, generationof a first and a second viewing window in a directional display deviceand including curved light extraction features, in accordance with thepresent disclosure;

FIG. 5 is a schematic diagram illustrating generation of a first viewingwindow in a directional display device including linear light extractionfeatures, in accordance with the present disclosure;

FIG. 6A is a schematic diagram illustrating one embodiment of thegeneration of a first viewing window in a time multiplexed directionaldisplay device, in accordance with the present disclosure;

FIG. 6B is a schematic diagram illustrating another embodiment of thegeneration of a second viewing window in a time multiplexed directionaldisplay device in a second time slot, in accordance with the presentdisclosure;

FIG. 6C is a schematic diagram illustrating another embodiment of thegeneration of a first and a second viewing window in a time multiplexeddirectional display device, in accordance with the present disclosure;

FIG. 7 is a schematic diagram illustrating an observer trackingautostereoscopic directional display device, in accordance with thepresent disclosure;

FIG. 8 is a schematic diagram illustrating a multi-viewer directionaldisplay device, in accordance with the present disclosure;

FIG. 9 is a schematic diagram illustrating a privacy directional displaydevice, in accordance with the present disclosure;

FIG. 10 is a schematic diagram illustrating in side view, the structureof a directional display device, in accordance with the presentdisclosure;

FIG. 11 is a schematic diagram illustrating a front view of a wedge typedirectional backlight, in accordance with the present disclosure;

FIG. 12 is a schematic diagram illustrating a side view of a wedge typedirectional display device, in accordance with the present disclosure;

FIG. 13 is a schematic diagram illustrating a control system for anobserver tracking directional display apparatus, in accordance with thepresent disclosure;

FIG. 14A is a schematic diagram illustrating a top view of a directionaldisplay including a directional backlight, in accordance with thepresent disclosure;

FIG. 14B is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane, inaccordance with the present disclosure;

FIG. 15A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane and amethod to adjust luminous fluxes of light sources in the array, inaccordance with the present disclosure;

FIG. 15B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources and method to adjust luminousfluxes of light sources in the array, in accordance with the presentdisclosure;

FIG. 15C is a schematic diagram illustrating a detail of FIG. 15B, inaccordance with the present disclosure;

FIG. 15D is a schematic diagram illustrating a graph of luminous fluxdistribution and a front view of a directional backlight aligned withthe luminous flux distribution, in accordance with the presentdisclosure;

FIG. 15E is a schematic diagram illustrating a graph of luminous fluxdistribution for an autostereoscopic Lambertian display system, inaccordance with the present disclosure;

FIG. 15F is a schematic diagram illustrating a graph of luminous fluxdistribution for an autostereoscopic display system with a gain ofgreater than one and an on-axis viewing position, in accordance with thepresent disclosure;

FIG. 15G is a schematic diagram illustrating a graph of luminous fluxdistribution for an autostereoscopic display system with a gain ofgreater than one and an off-axis viewing position, in accordance withthe present disclosure;

FIG. 15H is a schematic diagram illustrating a further graph of luminousflux distribution for an autostereoscopic display system with a gain ofgreater than one and an off-axis viewing position, in accordance withthe present disclosure;

FIG. 16 is a schematic diagram illustrating a light source addressingapparatus, in accordance with the present disclosure;

FIG. 17 is a schematic diagram illustrating a graph of luminousintensity for an array of optical windows and respective opticalwindows, in accordance with the present disclosure;

FIG. 18 is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position for a waveguide includinguniform luminous intensity of light sources, in accordance with thepresent disclosure;

FIG. 19A is a schematic diagram illustrating a perspective view of alight ray incident with a first direction onto a light extractionfeature of a waveguide, in accordance with the present disclosure;

FIG. 19B is a schematic diagram illustrating a perspective view of alight ray incident with a second direction onto a light extractionfeature of a waveguide, in accordance with the present disclosure;

FIG. 20A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide and a method to adjust luminous fluxes of light sources in thearray, in accordance with the present disclosure;

FIG. 20B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources and method to adjust luminousfluxes of light sources in the array for a waveguide, in accordance withthe present disclosure;

FIG. 21A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide and a method to adjust luminous fluxes of light sources in thearray, in accordance with the present disclosure;

FIG. 21B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources and method to adjust luminousfluxes of light sources in the array for a waveguide, in accordance withthe present disclosure;

FIG. 22 is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane and amethod to adjust luminous fluxes of light sources in the array for leftand right eye illumination phases, in accordance with the presentdisclosure;

FIG. 23A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide and a method to adjust luminous fluxes of light sources in thearray for the right eye illumination phase, in accordance with thepresent disclosure;

FIG. 23B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources for left and right eyeillumination phases and method to adjust luminous fluxes of lightsources in the array for a waveguide for the right eye illuminationphase, in accordance with the present disclosure;

FIG. 24 is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide, in accordance with the present disclosure;

FIG. 25 is a schematic diagram illustrating a further graph of opticalwindow luminous intensity against viewing position in the window planeof a waveguide, in accordance with the present disclosure;

FIG. 26A is a schematic diagram illustrating a further graph of opticalwindow luminous intensity against viewing position in the window planeof a waveguide, in accordance with the present disclosure;

FIG. 26B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources to compensate for lightsource degradation, in accordance with the present disclosure;

FIG. 27A is a schematic diagram illustrating a front view of adirectional display apparatus in landscape mode, in accordance with thepresent disclosure;

FIG. 27B is a schematic diagram illustrating a front view of adirectional display apparatus in portrait mode, in accordance with thepresent disclosure;

FIG. 27C is a schematic diagram illustrating a further graph of opticalwindow luminous intensity against viewing position in the window planeof a waveguide for the arrangement of FIG. 27B, in accordance with thepresent disclosure;

FIG. 28 is a schematic diagram illustrating a side view of a waveguideand viewing windows for observer movement, in accordance with thepresent disclosure;

FIG. 29 is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of light sources and method to adjust luminousfluxes of light sources in the array for a waveguide with the viewermovement of FIG. 28, in accordance with the present disclosure;

FIG. 30 is a schematic diagram illustrating an arrangement of viewingwindows during observer motion between viewing positions in accordancewith the present disclosure;

FIG. 31 is a schematic diagram illustrating intensity distribution of anarrangement of viewing windows in accordance with the presentdisclosure;

FIG. 32 is a schematic diagram illustrating a further intensitydistribution of an arrangement of viewing windows in accordance with thepresent disclosure;

FIG. 33 is a schematic diagram illustrating a further intensitydistribution of an arrangement of viewing windows in accordance with thepresent disclosure;

FIG. 34 is a schematic diagram illustrating a further intensitydistribution of an arrangement of viewing windows in accordance with thepresent disclosure;

FIG. 35 is a schematic diagram illustrating non-uniformities of displayillumination arising from a non-uniform window intensity distribution inaccordance with the present disclosure;

FIGS. 36-41 are schematic diagrams illustrating non-uniform luminousflux distributions from light sources, in accordance with the presentdisclosure;

FIG. 42 is a schematic diagram illustrating a 2D directional display inlandscape orientation, in accordance with the present disclosure;

FIG. 43 is a schematic diagram illustrating a graph of scaled luminousflux against light source position for the display of FIG. 42, inaccordance with the present disclosure;

FIG. 44 is a schematic diagram illustrating a 2D directional display inportrait orientation, in accordance with the present disclosure;

FIG. 45 is a schematic diagram illustrating a graph of scaled luminousflux against light source position for the display of FIG. 44, inaccordance with the present disclosure;

FIG. 46 is a schematic diagram illustrating a control system and frontview of a directional backlight apparatus, in accordance with thepresent disclosure;

FIG. 47 is a schematic diagram illustrating a control system and frontview of a directional backlight apparatus, in accordance with thepresent disclosure;

FIG. 48 is a schematic diagram illustrating an apparatus to drive alight source for a calibration mode of operation, in accordance with thepresent disclosure;

FIG. 49 is a schematic diagram illustrating a light source array incalibration mode of operation; in accordance with the presentdisclosure;

FIG. 50 is a schematic diagram illustrating a front view of a lightsource array arranged to achieve color correction, in accordance withthe present disclosure;

FIG. 51 is a schematic diagram illustrating a front view of a furtherlight source array arranged to achieve color correction, in accordancewith the present disclosure;

FIG. 52 is a schematic diagram illustrating a graph of optical windowchromaticity variations and a method to correct chromaticity variations,in accordance with the present disclosure;

FIG. 53 is a schematic diagram illustrating a front view of a lightsource array, in accordance with the present disclosure; and

FIG. 54 is a schematic diagram illustrating a front view of a lightsource array and method to correct a light source failure, in accordancewith the present disclosure.

DETAILED DESCRIPTION

Time multiplexed autostereoscopic displays can advantageously improvethe spatial resolution of autostereoscopic display by directing lightfrom all of the pixels of a spatial light modulator to a first viewingwindow in a first time slot, and all of the pixels to a second viewingwindow in a second time slot. Thus an observer with eyes arranged toreceive light in first and second viewing windows will see a fullresolution image across the whole of the display over multiple timeslots. Time multiplexed displays can advantageously achieve directionalillumination by directing an illuminator array through a substantiallytransparent time multiplexed spatial light modulator using directionaloptical elements, wherein the directional optical elements substantiallyform an image of the illuminator array in the window plane.

The uniformity of the viewing windows may be advantageously independentof the arrangement of pixels in the spatial light modulator.Advantageously, such displays can provide observer tracking displayswhich have low flicker, with low levels of cross talk for a movingobserver.

To achieve high uniformity in the window plane, it is desirable toprovide an array of illumination elements that have a high spatialuniformity. The illuminator elements of the time sequential illuminationsystem may be provided, for example, by pixels of a spatial lightmodulator with size approximately 100 micrometers in combination with alens array. However, such pixels suffer from similar difficulties as forspatially multiplexed displays. Further, such devices may have lowefficiency and higher cost, requiring additional display components.

High window plane uniformity can be conveniently achieved withmacroscopic illuminators, for example, an array of LEDs in combinationwith homogenizing and diffusing optical elements that are typically ofsize 1 mm or greater. However, the increased size of the illuminatorelements means that the size of the directional optical elementsincreases proportionately. For example, a 16 mm wide illuminator imagedto a 65 mm wide viewing window may require a 200 mm back workingdistance. Thus, the increased thickness of the optical elements canprevent useful application, for example, to mobile displays, or largearea displays.

Addressing the aforementioned shortcomings, optical valves as describedin commonly-owned U.S. patent application Ser. No. 13/300,293advantageously can be arranged in combination with fast switchingtransmissive spatial light modulators to achieve time multiplexedautostereoscopic illumination in a thin package while providing highresolution images with flicker free observer tracking and low levels ofcross talk. Described is a one dimensional array of viewing positions,or windows, that can display different images in a first, typicallyhorizontal, direction, but contain the same images when moving in asecond, typically vertical, direction.

Conventional non-imaging display backlights commonly employ opticalwaveguides and have edge illumination from light sources such as LEDs.However, it should be appreciated that there are many fundamentaldifferences in the function, design, structure, and operation betweensuch conventional non-imaging display backlights and the imagingdirectional backlights discussed in the present disclosure.

Generally, for example, in accordance with the present disclosure,imaging directional backlights are arranged to direct the illuminationfrom multiple light sources through a display panel to respectivemultiple optical windows in at least one axis. Each optical window issubstantially formed as an image in at least one axis of a light sourceby the imaging system of the imaging directional backlight. An imagingsystem may be formed to image multiple light sources to respectiveviewing windows. In this manner, the light from each of the multiplelight sources is substantially not visible for an observer's eye outsideof the respective viewing window.

In contradistinction, conventional non-imaging backlights or lightguiding plates (LGPs) are used for illumination of 2D displays. See,e.g., Kälil Käläntär et al., Backlight Unit With Double Surface LightEmission, J. Soc. Inf. Display, Vol. 12, Issue 4, pp. 379-387 (December2004). Non-imaging backlights are typically arranged to direct theillumination from multiple light sources through a display panel into asubstantially common viewing zone for each of the multiple light sourcesto achieve wide viewing angle and high display uniformity. Thusnon-imaging backlights do not form viewing windows. In this manner, thelight from each of the multiple light sources may be visible for anobserver's eye at substantially all positions across the viewing zone.Such conventional non-imaging backlights may have some directionality,for example, to increase screen gain compared to Lambertianillumination, which may be provided by brightness enhancement films suchas BEF™ from 3M. However, such directionality may be substantially thesame for each of the respective light sources. Thus, for these reasonsand others that should be apparent to persons of ordinary skill,conventional non-imaging backlights are different to imaging directionalbacklights. Edge lit non-imaging backlight illumination structures maybe used in liquid crystal display systems such as those seen in 2DLaptops, Monitors and TVs. Light propagates from the edge of a lossywaveguide which may include sparse features; typically localindentations in the surface of the guide which cause light to be lostregardless of the propagation direction of the light.

As used herein, an optical valve is an optical structure that may be atype of light guiding structure or device referred to as, for example, alight valve, an optical valve directional backlight, and a valvedirectional backlight (“v-DBL”). In the present disclosure, opticalvalve is different to a spatial light modulator (even though spatiallight modulators may be sometimes generally referred to as a “lightvalve” in the art). One example of an imaging directional backlight isan optical valve that may employ a folded optical system. Light maypropagate substantially without loss in one direction through theoptical valve, may be incident on an imaging reflector, and maycounter-propagate such that the light may be extracted by reflection offtilted light extraction features, and directed to viewing windows asdescribed in patent application Ser. No. 13/300,293, which is hereinincorporated by reference in its entirety.

As used herein, examples of an imaging directional backlight include astepped waveguide imaging directional backlight, a folded imagingdirectional backlight, a wedge type directional backlight, or an opticalvalve.

Additionally, as used herein, a stepped waveguide imaging directionalbacklight may be an optical valve. A stepped waveguide is a waveguidefor an imaging directional backlight including a waveguide for guidinglight, further including a first light guiding surface; and a secondlight guiding surface, opposite the first light guiding surface, furtherincluding a plurality of guiding features interspersed with a pluralityof extraction features arranged as steps.

Moreover, as used, a folded imaging directional backlight may be atleast one of a wedge type directional backlight, or an optical valve.

In operation, light may propagate within an exemplary optical valve in afirst direction from an input end to a reflective end and may betransmitted substantially without loss. Light may be reflected at thereflective end and propagates in a second direction substantiallyopposite the first direction. As the light propagates in the seconddirection, the light may be incident on light extraction features, whichare operable to redirect the light outside the optical valve. Stateddifferently, the optical valve generally allows light to propagate inthe first direction and may allow light to be extracted whilepropagating in the second direction.

The optical valve may achieve time sequential directional illuminationof large display areas. Additionally, optical elements may be employedthat are thinner than the back working distance of the optical elementsto direct light from macroscopic illuminators to a nominal window plane.Such displays may use an array of light extraction features arranged toextract light counter propagating in a substantially parallel waveguide.

Thin imaging directional backlight implementations for use with LCDshave been proposed and demonstrated by 3M, for example U.S. Pat. No.7,528,893; by Microsoft, for example U.S. Pat. No. 7,970,246 which maybe referred to herein as a “wedge type directional backlight;” by RealD,for example U.S. patent application Ser. No. 13/300,293 which may bereferred to herein as an “optical valve” or “optical valve directionalbacklight,” all of which are herein incorporated by reference in theirentirety.

The present disclosure provides stepped waveguide imaging directionalbacklights in which light may reflect back and forth between theinternal faces of, for example, a stepped waveguide which may include afirst guide surface and a second guide surface comprising a plurality oflight extraction features and intermediate regions. As the light travelsalong the length of the stepped waveguide, the light may notsubstantially change angle of incidence with respect to the first andsecond guide surfaces and so may not reach the critical angle of themedium at these internal surfaces. Light extraction may beadvantageously achieved by a light extraction features which may befacets of the second guide surface (the step “risers”) that are inclinedto the intermediate regions (the step “treads”). Note that the lightextraction features may not be part of the light guiding operation ofthe stepped waveguide, but may be arranged to provide light extractionfrom the structure. By contrast, a wedge type imaging directionalbacklight may allow light to guide within a wedge profiled waveguidehaving continuous internal surfaces. Thus, the stepped waveguide(optical valve) is thus not a wedge type imaging directional backlight.

FIG. 1A is a schematic diagram illustrating a front view of lightpropagation in one embodiment of a directional display device, and FIG.1B is a schematic diagram illustrating a side view of light propagationin the directional display device of FIG. 1A.

FIG. 1A illustrates a front view in the xy plane of a directionalbacklight of a directional display device, and includes an illuminatorarray 15 which may be used to illuminate a stepped waveguide 1.Illuminator array 15 includes illuminator elements 15 a throughilluminator element 15 n (where n is an integer greater than one). Inone example, the stepped waveguide 1 of FIG. 1A may be a stepped,display sized waveguide 1. Illuminator elements 15 a through 15 n arelight sources that may be light emitting diodes (LEDs). Although LEDsare discussed herein as illuminator elements 15 a-15 n, other lightsources may be used such as, but not limited to, diode sources,semiconductor sources, laser sources, local field emission sources,organic emitter arrays, and so forth. Additionally, FIG. 1B illustratesa side view in the xz plane, and includes illuminator array 15, SLM(spatial light modulator) 48, extraction features 12, intermediateregions 10, and stepped waveguide 1, arranged as shown. The side viewprovided in FIG. 1B is an alternative view of the front view shown inFIG. 1A. Accordingly, the illuminator array 15 of FIGS. 1A and 1Bcorresponds to one another and the stepped waveguide 1 of FIGS. 1A and1B may correspond to one another.

Further, in FIG. 1B, the stepped waveguide 1 may have an input end 2that is thin and a reflective end 4 that is thick. Thus the waveguide 1extends between the input end 2 that receives input light and thereflective end 4 that reflects the input light back through thewaveguide 1. The length of the input end 2 in a lateral direction acrossthe waveguide is greater than the height of the input end 2. Theilluminator elements 15 a-15 n are disposed at different input positionsin a lateral direction across the input end 2.

The waveguide 1 has first and second, opposed guide surfaces extendingbetween the input end 2 and the reflective end 4 for guiding lightforwards and back along the waveguide 1 by total internal reflection(TIR). The first guide surface is planar. The second guide surface has aplurality of light extraction features 12 facing the reflective end 4and inclined to reflect at least some of the light guided back throughthe waveguide 1 from the reflective end in directions that break thetotal internal reflection at the first guide surface and allow outputthrough the first guide surface, for example, upwards in FIG. 1B, thatis supplied to the SLM 48.

In this example, the light extraction features 12 are reflective facets,although other reflective features could be used. The light extractionfeatures 12 do not guide light through the waveguide, whereas theintermediate regions of the second guide surface intermediate the lightextraction features 12 guide light without extracting it. Those regionsof the second guide surface are planar and may extend parallel to thefirst guide surface, or at a relatively low inclination. The lightextraction features 12 extend laterally to those regions so that thesecond guide surface has a stepped shape including of the lightextraction features 12 and intermediate regions. The light extractionfeatures 12 are oriented to reflect light from the light sources, afterreflection from the reflective end 4, through the first guide surface.

The light extraction features 12 are arranged to direct input light fromdifferent input positions in the lateral direction across the input endin different directions relative to the first guide surface that aredependent on the input position. As the illumination elements 15 a-15 nare arranged at different input positions, the light from respectiveillumination elements 15 a-15 n is reflected in those differentdirections. In this manner, each of the illumination elements 15 a-15 ndirects light into a respective optical window in output directionsdistributed in the lateral direction in dependence on the inputpositions. The lateral direction across the input end 2 in which theinput positions are distributed corresponds with regard to the outputlight to a lateral direction to the normal to the first guide surface.The lateral directions as defined at the input end 2 and with regard tothe output light remain parallel in this embodiment where thedeflections at the reflective end 4 and the first guide surface aregenerally orthogonal to the lateral direction. Under the control of acontrol system, the illuminator elements 15 a-15 n may be selectivelyoperated to direct light into a selectable optical window. The opticalwindows may be used individually or in groups as viewing windows.

In the present disclosure an optical window may correspond to the imageof a single light source in the window plane, being a nominal plane inwhich optical windows form across the entirety of the display device.Alternatively, an optical window may correspond to the image of a groupsof light sources that are driven together. Advantageously, such groupsof light sources may increase uniformity of the optical windows of thearray 121.

By way of comparison, a viewing window is a region in the window planewherein light is provided comprising image data of substantially thesame image from across the display area. Thus a viewing window may beformed from a single optical window or from plural optical windows,under the control of the control system.

The SLM 48 extends across the waveguide is transmissive and modulatesthe light passing therethrough. Although the SLM 48 may be a liquidcrystal display (LCD) but this is merely by way of example, and otherspatial light modulators or displays may be used including LCOS, DLPdevices, and so forth, as this illuminator may work in reflection. Inthis example, the SLM 48 is disposed across the first guide surface ofthe waveguide and modulates the light output through the first guidesurface after reflection from the light extraction features 12.

The operation of a directional display device that may provide a onedimensional array of optical windows is illustrated in front view inFIG. 1A, with its side profile shown in FIG. 1B. In operation, in FIGS.1A and 1B, light may be emitted from an illuminator array 15, such as anarray of illuminator elements 15 a through 15 n, located at differentpositions, y, along the surface of input end 2, x=0, of the steppedwaveguide 1. The light may propagate along +x in a first direction,within the stepped waveguide 1, while at the same time, the light mayfan out in the xy plane and upon reaching the reflective end 4 that iscurved to have a positive optical power in the lateral direction, maysubstantially or entirely fill the curved end side 4. While propagating,the light may spread out to a set of angles in the xz plane up to, butnot exceeding the critical angle of the guide material. The extractionfeatures 12 that link the intermediate regions 10 of the second guidesurface of the stepped waveguide 1 may have a tilt angle greater thanthe critical angle and hence may be missed by substantially all lightpropagating along +x in the first direction, ensuring the substantiallylossless forward propagation.

Continuing the discussion of FIGS. 1A and 1B, the reflective end 4 ofthe stepped waveguide 1 may be made reflective, typically by beingcoated with a reflective material such as, for example, silver, althoughother reflective techniques may be employed. Light may therefore beredirected in a second direction, back down the guide in the directionof −x and may be substantially collimated in the xy or display plane.The angular spread may be substantially preserved in the xz plane aboutthe principal propagation direction, which may allow light to hit theriser edges and reflect out of the guide. In an embodiment withapproximately 45 degree tilted extraction features 12, light may beeffectively directed approximately normal to the xy display plane withthe xz angular spread substantially maintained relative to thepropagation direction. This angular spread may be increased when lightexits the stepped waveguide 1 through refraction, but may be decreasedsomewhat dependent on the reflective properties of the extractionfeatures 12.

In some embodiments with uncoated extraction features 12, reflection maybe reduced when total internal reflection (TIR) fails, squeezing the xzangular profile and shifting off normal. However, in other embodimentshaving silver coated or metallized extraction features, the increasedangular spread and central normal direction may be preserved. Continuingthe description of the embodiment with silver coated extractionfeatures, in the xz plane, light may exit the stepped waveguide 1approximately collimated and may be directed off normal in proportion tothe y-position of the respective illuminator element 15 a-15 n inilluminator array 15 from the input edge center. Having independentilluminator elements 15 a-15 n along the input end 2 then enables lightto exit from the entire first guide surface 6 and propagate at differentexternal angles, as illustrated in FIG. 1A.

Illuminating a spatial light modulator (SLM) 48 such as a fast liquidcrystal display (LCD) panel with such a device may achieveautostereoscopic 3D as shown in top view or yz-plane viewed from theilluminator array 15 end in FIG. 2A, front view in FIG. 2B and side viewin FIG. 2C. FIG. 2A is a schematic diagram illustrating in a top view,propagation of light in a directional display device, FIG. 2B is aschematic diagram illustrating in a front view, propagation of light ina directional display device, and FIG. 2C is a schematic diagramillustrating in side view propagation of light in a directional displaydevice. As illustrated in FIGS. 2A, 2B, and 2C, a stepped waveguide 1may be located behind a fast (e.g., greater than 100 Hz) LCD panel SLM48 that displays sequential right and left eye images. Insynchronization, specific illuminator elements 15 a through 15 n ofilluminator array 15 (where n is an integer greater than one) may beselectively turned on and off, providing illuminating light that entersright and left eyes substantially independently by virtue of thesystem's directionality. In the simplest case, sets of illuminatorelements of illuminator array 15 are turned on together, providing a onedimensional viewing window 26 or an optical pupil with limited width inthe horizontal direction, but extended in the vertical direction, inwhich both eyes horizontally separated may view a left eye image, andanother viewing window 44 in which a right eye image may primarily beviewed by both eyes, and a central position in which both the eyes mayview different images. Viewing window 26 may comprise an array ofoptical windows 260 and viewing window 44 may comprise an array ofoptical windows 440, wherein each optical window is formed by a singleilluminator of the array 15. Thus multiple illuminators may be arrangedto form viewing windows 26 and 44. In FIG. 2A, the viewing window 26 isshown as formed by a single illuminator 15 a and may thus comprise asingle optical window 260. Similarly, the viewing window 44 is shown asformed by a single illuminator 15 n and may thus comprise a singleoptical window 440. In this way, 3D may be viewed when the head of aviewer is approximately centrally aligned. Movement to the side awayfrom the central position may result in the scene collapsing onto a 2Dimage.

The reflective end 4 may have positive optical power in the lateraldirection across the waveguide. In embodiments in which typically thereflective end 4 has positive optical power, the optical axis may bedefined with reference to the shape of the reflective end 4, for examplebeing a line that passes through the centre of curvature of thereflective end 4 and coincides with the axis of reflective symmetry ofthe end 4 about the x-axis. In the case that the reflecting surface 4 isflat, the optical axis may be similarly defined with respect to othercomponents having optical power, for example the light extractionfeatures 12 if they are curved, or the Fresnel lens 62 described below.The optical axis 238 is typically coincident with the mechanical axis ofthe waveguide 1. In the present embodiments that typically comprise asubstantially cylindrical reflecting surface at end 4, the optical axis238 is a line that passes through the centre of curvature of the surfaceat end 4 and coincides with the axis of reflective symmetry of the side4 about the x-axis. The optical axis 238 is typically coincident withthe mechanical axis of the waveguide 1. The cylindrical reflectingsurface at end 4 may typically be a spherical profile to optimizeperformance for on-axis and off-axis viewing positions. Other profilesmay be used.

FIG. 3 is a schematic diagram illustrating in side view a directionaldisplay device. Further, FIG. 3 illustrates additional detail of a sideview of the operation of a stepped waveguide 1, which may be atransparent material. The stepped waveguide 1 may include an illuminatorinput end 2, a reflective end 4, a first guide surface 6 which may besubstantially planar, and a second guide surface 8 which includesintermediate regions 10 and light extraction features 12. In operation,light rays 16 from an illuminator element 15 c of an illuminator array15 (not shown in FIG. 3), that may be an addressable array of LEDs forexample, may be guided in the stepped waveguide 1 by means of totalinternal reflection by the first guide surface 6 and total internalreflection by the intermediate regions 10 of the second guide surface 8,to the reflective end 4, which may be a mirrored surface. Althoughreflective end 4 may be a mirrored surface and may reflect light, it mayin some embodiments also be possible for light to pass throughreflective end 4.

Continuing the discussion of FIG. 3, light ray 18 reflected by thereflective end 4 may be further guided in the stepped waveguide 1 bytotal internal reflection at the reflective end 4 and may be reflectedby extraction features 12. Light rays 18 that are incident on extractionfeatures 12 may be substantially deflected away from guiding modes ofthe stepped waveguide 1 and may be directed, as shown by ray 20, throughthe first guide surface 6 to an optical pupil that may form a viewingwindow 26 of an autostereoscopic display. The width of the viewingwindow 26 may be determined by at least the size of the illuminator,number of illuminator elements 15 n illuminated, output design distanceand optical power in the reflective end 4 and extraction features 12.The height of the viewing window may be primarily determined by thereflection cone angle of the extraction features 12 and the illuminationcone angle input at the input end 2. Thus each viewing window 26represents a range of separate output directions with respect to thesurface normal direction of the spatial light modulator 48 thatintersect with a window plane 106 at the nominal viewing distance.

FIG. 4A is a schematic diagram illustrating in front view a directionaldisplay device which may be illuminated by a first illuminator elementand including curved light extraction features. Further, FIG. 4A showsin front view further guiding of light rays from illuminator element 15c of illuminator array 15, in the stepped waveguide 1 having an opticalaxis 28. In FIG. 4A, the directional backlight may include the steppedwaveguide 1 and the light source illuminator array 15. Each of theoutput rays are directed from the input end 2 towards the same opticalwindow 260 from the respective illuminator 15 c. The light rays of FIG.4A may exit the reflective end 4 of the stepped waveguide 1. As shown inFIG. 4A, ray 16 may be directed from the illuminator element 15 ctowards the reflective end 4. Ray 18 may then reflect from a lightextraction feature 12 and exit the reflective end 4 towards the opticalwindow 260. Thus light ray 30 may intersect the ray 20 in the opticalwindow 260, or may have a different height in the viewing window asshown by ray 32. Additionally, in various embodiments, sides 22, 24 ofthe waveguide 1 may be transparent, mirrored, or blackened surfaces.Continuing the discussion of FIG. 4A, light extraction features 12 maybe elongate, and the orientation of light extraction features 12 in afirst region 34 of the second guide surface 8 (shown in FIG. 3, but notshown in FIG. 4A) may be different to the orientation of lightextraction features 12 in a second region 36 of the second guide surface8. Similar to other embodiments discussed herein, for example asillustrated in FIG. 3, the light extraction features of FIG. 4A mayalternate with the intermediate regions 10. As illustrated in FIG. 4A,the stepped waveguide 1 may include a reflective surface on reflectiveend 4. In one embodiment, the reflective end of the stepped waveguide 1may have positive optical power in a lateral direction across thestepped waveguide 1.

In another embodiment, the light extraction features 12 of eachdirectional backlight may have positive optical power in a lateraldirection across the waveguide.

In another embodiment, each directional backlight may include lightextraction features 12 which may be facets of the second guide surface.The second guide surface may have regions alternating with the facetsthat may be arranged to direct light through the waveguide withoutsubstantially extracting it.

FIG. 4B is a schematic diagram illustrating in front view a directionaldisplay device which may illuminated by a second illuminator element.Further, FIG. 4B shows the light rays 40, 42 from a second illuminatorelement 15 h of the illuminator array 15. The curvature of thereflective surface on the reflective end 4 and the light extractionfeatures 12 cooperatively produce a second optical window 440 laterallyseparated from the optical window 260 with light rays from theilluminator element 15 h.

Advantageously, the arrangement illustrated in FIG. 4B may provide areal image of the illuminator element 15 c at an optical window 260 inwhich the real image may be formed by cooperation of optical power inreflective end 4 and optical power which may arise from differentorientations of elongate light extraction features 12 between regions 34and 36, as shown in FIG. 4A. The arrangement of FIG. 4B may achieveimproved aberrations of the imaging of illuminator element 15 c tolateral positions in optical window 260. Improved aberrations mayachieve an extended viewing freedom for an autostereoscopic displaywhile achieving low cross talk levels.

FIG. 5 is a schematic diagram illustrating in front view an embodimentof a directional display device having substantially linear lightextraction features. Further, FIG. 5 shows a similar arrangement ofcomponents to FIG. 1 (with corresponding elements being similar), withone of the differences being that the light extraction features 12 aresubstantially linear and parallel to each other. Advantageously, such anarrangement may provide substantially uniform illumination across adisplay surface and may be more convenient to manufacture than thecurved extraction features of FIG. 4A and FIG. 4B. The optical axis 321of the directional waveguide 1 may be the optical axis direction of thesurface at the reflective end 4. The optical power of the reflective end4 is arranged to be across the optical axis direction, thus raysincident on the reflective end 4 will have an angular deflection thatvaries according to the lateral offset 319 of the incident ray from theoptical axis 321.

FIG. 6A is a schematic diagram illustrating one embodiment of thegeneration of a first viewing window in a time multiplexed imagingdirectional display device in a first time slot, FIG. 6B is a schematicdiagram illustrating another embodiment of the generation of a secondviewing window in a time multiplexed imaging directional backlightapparatus in a second time slot, and FIG. 6C is a schematic diagramillustrating another embodiment of the generation of a first and asecond viewing window in a time multiplexed imaging directional displaydevice. Further, FIG. 6A shows schematically the generation of viewingwindow 26 from stepped waveguide 1. Illuminator element group 31 inilluminator array 15 may provide a light cone 17 directed towards aviewing window 26 (that may comprise a single optical window 260 or anarray of optical windows 260). FIG. 6B shows schematically thegeneration of viewing window 44. Illuminator element group 33 inilluminator array 15 may provide a light cone 19 directed towardsviewing window 44 (that may comprise a single optical window 440 or anarray of optical windows 440). In cooperation with a time multiplexeddisplay, viewing windows 26 and 44 may be provided in sequence as shownin FIG. 6C. If the image on a spatial light modulator 48 (not shown inFIGS. 6A, 6B, 6C) is adjusted in correspondence with the light directionoutput, then an autostereoscopic image may be achieved for a suitablyplaced viewer. Similar operation can be achieved with all the imagingdirectional backlights described herein. Note that illuminator elementgroups 31, 33 each include one or more illumination elements fromillumination elements 15 a to 15 n, where n is an integer greater thanone.

FIG. 7 is a schematic diagram illustrating one embodiment of an observertracking autostereoscopic display apparatus including a time multiplexeddirectional display device. As shown in FIG. 7, selectively turning onand off illuminator elements 15 a to 15 n along axis 29 provides fordirectional control of viewing windows 26, 44. The head 45 position maybe monitored with a camera, motion sensor, motion detector, or any otherappropriate optical, mechanical or electrical means, and the appropriateilluminator elements of illuminator array 15 may be turned on and off toprovide substantially independent images to each eye irrespective of thehead 45 position. The head tracking system (or a second head trackingsystem) may provide monitoring of more than one head 45, 47 (head 47 notshown in FIG. 7) and may supply the same left and right eye images toeach viewers' left and right eyes providing 3D to all viewers. Againsimilar operation can be achieved with all the imaging directionalbacklights described herein.

FIG. 8 is a schematic diagram illustrating one embodiment of amulti-viewer directional display device as an example including animaging directional backlight. As shown in FIG. 8, at least two 2Dimages may be directed towards a pair of viewers 45, 47 so that eachviewer may watch a different image on the spatial light modulator 48.The two 2D images of FIG. 8 may be generated in a similar manner asdescribed with respect to FIG. 7 in that the two images would bedisplayed in sequence and in synchronization with sources whose light isdirected toward the two viewers. One image is presented on the spatiallight modulator 48 in a first phase, and a second image is presented onthe spatial light modulator 48 in a second phase different from thefirst phase. In correspondence with the first and second phases, theoutput illumination is adjusted to provide first and second viewingwindows 26, 44 respectively. An observer with both eyes in viewingwindow 26 will perceive a first image while an observer with both eyesin viewing window 44 will perceive a second image.

FIG. 9 is a schematic diagram illustrating a privacy directional displaydevice which includes an imaging directional backlight. 2D displaysystems may also utilize directional backlighting for security andefficiency purposes in which light may be primarily directed at the eyesof a first viewer 45 as shown in FIG. 9. Further, as illustrated in FIG.9, although first viewer 45 may be able to view an image on device 50,light is not directed towards second viewer 47. Thus second viewer 47 isprevented from viewing an image on device 50. Each of the embodiments ofthe present disclosure may advantageously provide autostereoscopic, dualimage or privacy display functions.

FIG. 10 is a schematic diagram illustrating in side view the structureof a time multiplexed directional display device as an example includingan imaging directional backlight. Further, FIG. 10 shows in side view anautostereoscopic directional display device, which may include thestepped waveguide 1 and a Fresnel lens 62 arranged to provide theviewing window 26 for a substantially collimated output across thestepped waveguide 1 output surface. A vertical diffuser 68 may bearranged to extend the height of the viewing window 26 further and toachieve blurring in directions in the vertical direction (parallel tothe x-axis) while minimizing blurring in directions in the lateraldirection (y axis). The light may then be imaged through the spatiallight modulator 48. The illuminator array 15 may include light emittingdiodes (LEDs) that may, for example, be phosphor converted blue LEDs, ormay be separate RGB LEDs. Alternatively, the illuminator elements inilluminator array 15 may include a uniform light source and spatiallight modulator arranged to provide separate illumination regions.Alternatively the illuminator elements may include laser lightsource(s). The laser output may be directed onto a diffuser by means ofscanning, for example, using a galvo or MEMS scanner. In one example,laser light may thus be used to provide the appropriate illuminatorelements in illuminator array 15 to provide a substantially uniformlight source with the appropriate output angle, and further to providereduction in speckle. Alternatively, the illuminator array 15 may be anarray of laser light emitting elements. Additionally in one example, thediffuser may be a wavelength converting phosphor, so that illuminationmay be at a different wavelength to the visible output light.

Thus, FIGS. 1 to 10 variously describe: a waveguide 1; a directionalbacklight comprising such a waveguide 1 and an illuminator array 15; anda directional display device including such a directional backlight andan SLM 48. As such the various features disclosed above with referenceto FIGS. 1 to 10 may be combined in any combination.

FIG. 11 is a schematic diagram illustrating a front view of anotherimaging directional backlight, as illustrated, a wedge type directionalbacklight, and FIG. 12 is a schematic diagram illustrating a side viewof a similar wedge type directional display device. A wedge typedirectional backlight is generally discussed by U.S. Pat. No. 7,660,047and entitled “Flat Panel Lens,” which is herein incorporated byreference in its entirety. The structure may include a wedge typewaveguide 1104 with a bottom surface which may be preferentially coatedwith a reflecting layer 1106 and with an end corrugated surface 1102,which may also be preferentially coated with a reflecting layer 1106.

In one embodiment, a directional display device may include a waveguidehaving an input end, first and second opposed guide surfaces for guidinglight along the waveguide, and a reflective end facing the input end forreflecting light from the input light back through the waveguide. Thedirectional display device may also include an array of light sourcesdisposed at different input positions across the input end of thewaveguide. The waveguide may be arranged to direct input light from thelight sources as output light through the first guide surface afterreflection from the reflective end into optical windows in outputdirections relative to the normal to the first guide surface and may beprimarily dependent on the input positions. The directional displaydevice may also include a transmissive spatial light modulator arrangedto receive the output light from the first guide surface and arranged tomodulate a first polarization component of the output light having afirst polarization.

In one embodiment of a wedge type directional backlight, the first guidesurface may be arranged to guide light by total internal reflection andthe second guide surface may be substantially planar and inclined at anangle to reflect light in directions that break the total internalreflection for outputting light through the first guide surface. Thewedge type directional backlight may be part of a directional displaydevice. The directional display device may also include a deflectionelement extending across the first guide surface of the waveguide fordeflecting light towards the normal to the spatial light modulator.

As shown in FIG. 12, light may enter the wedge type waveguide 1104 fromlocal sources 1101 and the light may propagate in a first directionbefore reflecting off the end surface. Light may exit the wedge typewaveguide 1104 while on its return path and may illuminate a displaypanel 1110. By way of comparison with an optical valve, a wedge typewaveguide provides extraction by a taper that reduces the incidenceangle of propagating light so that when the light is incident at thecritical angle on an output surface, it may escape. Escaping light atthe critical angle in the wedge type waveguide propagates substantiallyparallel to the surface until deflected by a redirection layer 1108 suchas a prism array. Errors or dust on the wedge type waveguide outputsurface may change the critical angle, creating stray light anduniformity errors. Further, an imaging directional backlight that uses amirror to fold the beam path in the wedge type directional backlight mayemploy a faceted mirror that biases the light cone directions in thewedge type waveguide. Such faceted mirrors are generally complex tofabricate and may result in illumination uniformity errors as well asstray light.

The wedge type directional backlight and optical valve further processlight beams in different ways. In the wedge type waveguide, light inputat an appropriate angle will output at a defined position on a majorsurface, but light rays will exit at substantially the same angle andsubstantially parallel to the major surface. By comparison, light inputto a stepped waveguide of an optical valve at a certain angle may outputfrom points across the first side, with output angle determined by inputangle. Advantageously, the stepped waveguide of the optical valve maynot require further light re-direction films to extract light towards anobserver and angular non-uniformities of input may not providenon-uniformities across the display surface.

However, in the present disclosure, in general a wedge type waveguidesuch as wedge type waveguide 1104 may be used in a directionalbacklight, and may replace the stepped waveguide 1 in the variousstructures shown in FIGS. 1 to 10 described above and in the structuresdescribed below.

There follows a description of some directional display apparatusesincluding a directional display device and a control system, wherein thedirectional display device includes a directional backlight including awaveguide and an SLM. In the following description, the waveguides,directional backlights and directional display devices are based on andincorporate the structures of FIGS. 1 to 10 above, but could equally beadapted to replace the stepped waveguide 1 by a wedge type waveguide asdescribed above. Except for the modifications and/or additional featureswhich will now be described, the above description applies equally tothe following waveguides, directional backlights and display devices,but for brevity will not be repeated.

FIG. 13 is a schematic diagram illustrating a directional displayapparatus including a display device 100 and a control system. Thearrangement and operation of the control system will now be describedand may be applied, with changes as necessary, to each of the displaydevices disclosed herein.

The directional display device 100 includes a directional backlight thatincludes waveguide 1 and an array 15 of illuminator elements 15 narranged as described above. The control system is arranged toselectively operate the illumination elements 15 a-15 n to direct lightinto selectable optical windows.

The waveguide 1 is arranged as described above. The reflective end 4converges the reflected light. A Fresnel lens 62 may be arranged tocooperate with reflective end 4 to achieve viewing windows 26 at awindow plane 106 observed by an observer 99. A transmissive spatiallight modulator (SLM) 48 may be arranged to receive the light from thedirectional backlight. Further a diffuser 68 may be provided tosubstantially remove Moiré beating between the waveguide 1 and pixels ofthe SLM 48 as well as the Fresnel lens 62.

The control system may include a sensor system arranged to detect theposition of the observer 99 relative to the display device 100. Thesensor system includes a position sensor 70, such as a camera, and ahead position measurement system 72 that may for example include acomputer vision image processing system. The control system may furtherinclude an illumination controller 74 and an image controller 76 thatare both supplied with the detected position of the observer suppliedfrom the head position measurement system 72. The illuminationcontroller 74 supplies drive signals to the illuminator elements 15 n.By controlling the drive signals, the illumination controller 74selectively operates the illuminator elements 15 n to direct light tointo the viewing windows 26 in cooperation with waveguide 1. Theillumination controller 74 selects the illuminator elements 15 n to beoperated in dependence on the position of the observer detected by thehead position measurement system 72, so that the viewing windows 26 intowhich light is directed are in positions corresponding to the left andright eyes of the observer 99. In this manner, the lateral outputdirectionality of the waveguide 1 corresponds with the observerposition.

The illumination controller 74 may be arranged to vary the drive signalssupplied to respective illuminator element 15 to control the luminousflux of the light emitted by the respective light sources of the array15, which may be referred to as the grey level of light. The luminousflux of a light source is a measure of the optical power emitted by thelight source, measured in lumens.

Control of the luminous flux may be effected by any suitable drivescheme including, without limitation, voltage modulation, currentmodulation, pulse width modulation, control of a spatial light modulatorarranged between a light source and the input end 2, or other knowngreyscale drive scheme. Further the luminous flux emitted by eachilluminator element 15 n may be varied by altering the current flowingin each addressable device, or by a pulse width modulation scheme wherethe length of one or more pulses is changed in order to vary thebrightness perceived by the observer due to the persistence of vision.It is also possible to combine these two effects to achieve thebrightness control desired.

The image controller 76 controls the SLM 48 to display images. Toprovide an autostereoscopic display, the image controller 76 and theillumination controller 74 may operate as follows. The image controller76 controls the SLM 48 to display temporally multiplexed left and righteye images. The illumination controller 74 operate the light sources 15to direct light into viewing windows in positions corresponding to theleft and right eyes of an observer synchronously with the display ofleft and right eye images. The position of the viewing windows mayprimarily depend on the detected position of the observer. In thismanner, an autostereoscopic effect is achieved using a time divisionmultiplexing technique.

As will now be described, a luminous flux controller 580, operatingunder user or automatic control, may control the illumination controller74 to implement a method of controlling the illuminator elements 15 n ofthe directional backlight to output light with luminous fluxes, scaledinversely by the width associated with the respective illuminatorelements 15 n in the lateral direction, that varies across the array ofilluminator elements 15.

The quantity of luminous flux that is varied across the array 15 is theluminous flux of an individual illuminator 15 that is scaled. Thescaling is inverse with the width associated with the respectiveilluminator elements 15 n in the lateral direction. The purpose of thatscaling is to take account of any variation in the pitch of theilluminator elements 15 n across the array 15. Thus, in the simple casethat the illuminator elements 15 n are disposed at different inputpositions with a constant pitch in the lateral direction across theinput end 4 of the waveguide 1, then the scaled luminous fluxes aresimply the actual luminous fluxes of the illuminator elements 15 n,because the scaling is constant. In cases where the pitch of theilluminator elements 15 n in the lateral direction is variable, then thescaled luminous fluxes take into account that varying pitch.

The scaled luminous fluxes may therefore may take into account gapsbetween the light sources and non-uniformities across luminous flux ofthe respective outputs. Thus, the width associated with an illuminatorelement 15 n may be taken as the pitch of the array illuminator elements15 n at the illuminator element 15 n being considered. Similarly thewidth associated with an illuminator element 15 n may be taken as thewidth between the mid-points of the gaps between the illuminatorelements 15 n. The scaled luminous flux is described further inreference to FIG. 53.

Herein, the scaled luminous fluxes are considered, because in thedisplay device 100 this quantity affects the luminous intensity of theoutput light as described further below. The luminous intensity of thedisplay device is a measure of the power emitted by the display devicein a particular direction per unit solid angle. Therefore the scaledluminous fluxes are controlled to provide a desired luminous intensity.

This is advantageous because the brightness of the display device 100 asperceived by the observer 99 is elicited by the luminance which is aphotometric measure of the luminous intensity per unit area of lighttraveling in a given direction. Thus variation of the luminous fluxlinear density allows the perceived brightness to be controlled, forexample allowing the perceived brightness (luminance) to be varied fordifferent positions of the observer 99 and/or power consumption to beminimized for a given perceived brightness.

The luminous flux under consideration is the total luminous fluxemitted. This may be derived by integrating the luminous flux emitted bythe illuminator elements 15 n over the direction perpendicular to thelateral direction.

In some embodiments, the luminous flux controller 580 may control theluminous flux to vary across the array of illuminator elements 15 n in aluminous flux distribution that is fixed with respect the position inthe lateral direction.

In other embodiments, the luminous flux controller 580 may control theluminous flux to vary across the array of illuminator elements 15 n independence on the detected position of the observer 99, as detected bythe sensor system.

Some specific ways in which the luminous flux controller 580 may controlthe luminous flux to vary across the array of illuminator elements 15 nwill now be described.

There will first be described some embodiments in which the luminousflux controller 580 controls the scaled luminous fluxes to vary acrossthe array of illuminator elements 15 n in a luminous flux distributionthat is fixed with respect the position in the lateral direction. Thishas particular advantage when the display device 100 is operated todisplay a 2D image viewable from a wide angle compared to a directionalmode of operation such as an autostereoscopic mode. In that case, allthe illuminator elements 15 n may simultaneously be operated, in whichcase the sensor system might be unused or omitted.

Alternatively, the illumination controller 74 may operate the lightsources 15 to direct light into a single viewing window visible by botheyes in dependence on the detected position of the observer with the SLM48 arranged to operate in a single phase for 2D viewing for privacy andhigh efficiency modes of operation. Such a viewing window issufficiently wide to be seen by both the left and right eyes of anobserver. Alternatively, the fixed luminous flux distribution may alsobe applied when the display device is operated to provide anautostereoscopic 3D display.

FIG. 14A is a schematic diagram illustrating a top view of a directionalbacklight that includes waveguide 1 and an array of illuminator elements15 n that provide an array of optical windows 260 in a window plane 106,the output direction and hence nominal lateral position in the windowplane of each optical window 260 being dependant on the lateral positionof the respective illuminator in the array 15. Thus at angle θcorresponding to a position 262 in the lateral (y-axis) direction acrossthe window plane 106, the luminous intensity of the display seen fromthe optical windows 260 may vary with position of an observer 99 (withright and left eye positions 560, 562 as indicated). It would bedesirable to achieve a variation of display luminous intensity that isLambertian so that the display appears to equally bright to each eye ofobserver 99; otherwise stated the luminance of the display issubstantially the same within the range of lateral viewing freedom. Itwould alternatively be desirable to achieve a variation of displayluminous intensity that achieves a reduced luminance for off-axisviewing positions to achieve power saving advantages.

The transfer function between each element of the array 15 ofilluminator elements 15 n and each optical window 260 of the array ofoptical windows will comprise the effects of scatter, diffusion,diffraction and imaging properties of the optical components arrangedbetween the array 15 and window plane 106. Thus the optical windows 260are not perfect images of the illuminator elements 15 n. However thelateral position 262 of the optical windows 260 will typically bedirectly related to the lateral position 261 of the illuminator elements15 n in the array 15.

Thus, the luminous flux controller 580 may control the scaled luminousfluxes to vary across the array of illuminator elements 15 n in aluminous flux distribution with the input position of the illuminatorelement 15 in the lateral direction, that produces a desired luminanceintensity distribution that represents the variation of the luminousintensity of the output light with the angle of the output directions,examples of which will now be described.

FIG. 14B is a schematic diagram illustrating a graph of the luminousintensity 264 of the output light against viewing position 262 in thewindow plane 106 corresponding to the angle θ of the output directions.Luminous intensity distribution 266 is Lambertian, having a luminousintensity that varies as the cosine of viewing angle θ and so theluminous intensity across the display may vary, while the observedluminance of the display is constant from viewing positions 500 to 502within the viewing windows.

In the present embodiments, a Lambertian emitter achieves the sameapparent luminance of a surface independent of the observer's angle ofview. Thus the surface has an isotropic luminance (measured in candelaper metre², or lumen per steradian per metre²) and the variation ofluminous intensity (measured in candela, or lumen per steradian) 264obeys Lambert's cosine law wherein luminous intensity observed from anideal diffuse radiator is directly proportional to the cosine of theangle θ between the observer's line of sight and the surface normal. Inthe present embodiments, Lambertian is used to describe the emission ofa display over a defined angular range, for example the total width ofoptical windows of the display. Thus at positions in the window planeoutside the width of the optical windows, the luminous intensitydistribution may behave in a non-Lambertian manner.

The luminous intensity may typically be considered for a point on thebacklight, for example the point corresponding to the centre of the SLM48. The luminance (luminous intensity per unit area) of the displaysystem may vary across the display area due to non-uniformities ofoutput and will further vary with subtended viewing angle of therespective unit area.

FIG. 14B further shows a luminous intensity distribution for a luminousintensity distribution 272 having a gain greater than one. Inparticular, the luminous intensity distribution 272 has a maximumluminous intensity, corresponding to the global maximum of the luminousflux distribution, that is greater than the luminous intensitydistribution 266 that is Lambertian, wherein the total power of the twoluminous intensity distributions 266 and 272 is the same over all outputdirections 500 to 502. Thus the peak luminous intensity is greater foron-axis positions, and falls at a faster rate than the luminousintensity distribution 266 that is Lambertian. Thus, the displayluminance falls for off-axis viewing positions. The ratio of luminanceof distribution 272 to peak luminance of distribution 266 is oftenreferred to as the gain of a display system.

The luminous flux controller 580 may control the scaled luminous fluxesto vary across the array of illuminator elements 15 n in a luminous fluxdistribution that provides the output light with a luminous intensitydistribution 266 that is Lambertian or with a luminous intensitydistribution 272 having a gain greater than one, as follows.

FIG. 15A is a schematic diagram illustrating a graph of luminousintensity of the output light against viewing position 262 in the windowplane 106 corresponding to the lateral angle θ of the output directionwith respect to the optical axis 238, as well as a method to adjust thescaled luminous fluxes of array 15. The luminous intensity distributions266 and 272 are shown but with their peak luminous intensities matchedso that the on-axis luminances are matched and the display appearsequally bright for on-axis viewing positions.

In one embodiment, the luminous intensity distribution 266 that isLambertian may be achieved by controlling all of the illuminatorelements of array 15 to have substantially the same scaled luminous fluxoutput.

To achieve the luminous intensity distribution 272, the luminousintensity for off-axis positions may be reduced in comparison to thedistribution 266, as indicated by arrows 270. This may be achieved bycontrolling the scaled luminous fluxes of the illuminator elements tovary across the array 15 in luminous flux distribution as follows.

FIG. 15B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of illuminator elements and method to adjustthe scaled luminous fluxes of the array 15. Thus the scaled luminousfluxes 263 may be plotted against the lateral position 261 in thelateral direction across the input end 2 of the waveguide 1. In aconstruction of the waveguide 1 in which the light extraction feature 12are mirrored, the scaled luminous fluxes 263 has a constant luminousflux distribution 269 to provide the luminous intensity distribution 266that is Lambertian. Arrows 271 shows the drop in scaled luminous flux,compared to constant luminous flux distribution 269, for respectivelight sources corresponding to arrow 270 at respective optical windowposition 262, to reach a non-linear luminous flux distribution 273 thatprovides the luminous intensity distribution 272 having a gain greaterthan one. Thus, the luminous flux distribution 273 has a global maximum508 of luminous flux and reduces on either side of the global maximum508. The global maximum 508 occurs in respect of the illuminatorelements aligned with the optical axis of the waveguide 1. In oneembodiment, illuminator elements of the array 15 may be controlled sothat their scaled luminous fluxes vary in accordance with the non-linearluminous flux distribution 273, thereby providing the display device 100with the luminous intensity distribution 272 having a gain greater thanone. That is, the luminous intensity 264 of the output light varies withthe angle 262 of the output directions in an actual luminous intensitydistribution 272 that is greater than a notional luminous intensitydistribution 266 that is Lambertian and has the same total luminousintensity over all output directions as the actual luminous intensitydistribution 272.

FIG. 15C is a schematic diagram illustrating a detail of FIG. 15B, butwith the y-axis plotting the actual luminous flux 265 integrated overthe direction perpendicular to the lateral direction. As mentionedabove, the actual luminous flux 265 at a given lateral position 261across the input end 2 is the integral of light captured by thewaveguide in a slice perpendicular to the lateral direction, i.e.parallel to the z-axis. Assuming that all of the light from therespective source is captured by the waveguide 1, the actual luminousflux 265 is the same as the integrated flux output of a slice of thelight source in the z-axis. As shown in FIG. 15C for detail 293 of FIG.15B, the light sources of the array 15 will typically have substantiallynon-uniform distribution of actual luminous flux 265 in the lateraldirection (y-axis) due to gaps between light sources and structurewithin the light sources. For example, as will be described in FIGS. 50and 51 the light source may comprise an LED with blue and yellow lightemitting regions. Different actual luminous flux 265 and chromaticityoutputs in the lateral direction when integrated across the thickness ofthe input end 2 may thus be achieved. For the present purposes, thelight sources may be arranged in a stepwise manner so that light sourcenumber 504 is related to a given luminous flux 506 providing the scaledluminous fluxes with the luminous flux distribution 273. Thus luminousflux distribution 273, may be provided by the actual luminous flux 265averaged over the width in the lateral direction associated with asingle illuminator element 15 n. In operation, the illuminator elements15 n are not imaged precisely in the window plane 106 so that theviewing windows may include laterally diffused overlapping opticalwindows. Thus the transformation from luminous flux distribution 273 toluminous intensity distribution 272 may further take into accountblurring between adjacent optical windows 260 within the viewing windows26, and may be arranged to achieve the desired luminous intensitydistribution 262.

Advantageously the output luminance of the display may be maintained foron-axis viewing positions, and reduced for off-axis viewing positions.Thus the power consumption of the display may be reduced for lessadvantageous viewing positions, improving display efficiency, batterycharge lifetime and reducing display cost.

FIG. 15D is a schematic diagram illustrating a graph of luminous fluxdistribution and a front view of a directional backlight aligned withthe luminous flux distribution. For illustrative convenience, luminousflux distribution 273 of the scaled luminous fluxes 263 with lateralposition 261 is shown aligned with respective illuminator elements ofarray 15. Thus illuminator elements 514, 516 have respective scaledluminous fluxes 510, 512, including the global maximum 508 ofdistribution 273 that is aligned with the optical axis 232 of thewaveguide 1. The distribution 273 is provided between positions 261 of501, 503. Thus the global maximum of the flux distribution may be inrespect of the illuminator elements aligned with the optical axis 232 ofthe waveguide. The global maximum of the flux distribution may be inrespect of the illuminator elements 514, 516 aligned with the opticalaxis 232 of the waveguide 1. Advantageously, the peak luminous intensityof output is provided on-axis with respect to the display normal 107that may be aligned with the optical axis 232. Thus the luminance isgreatest for the on-axis position for displays with gains greater thanone. The on-axis position is typically the desirable viewing position,particularly for mobile displays and so the display will look brightestfor the optimum viewing conditions while achieving reduced powerconsumption for off-axis viewing positions.

The global maximum 508 of the luminous flux distribution may be inrespect of the illuminator elements 514, 516 aligned with the opticalaxis 232 of the waveguide 1. Such an arrangement may achieve a globalmaximum 259 of luminous intensity 264, which may be for respectiveoptical windows that are aligned with the optical axis 232 of thewaveguide 1.

The embodiment of FIG. 15D illustrates one arrangement of illuminatorelements for wide angle 2D viewing; that is the illuminator elements mayoperate continuously or in a single phase and the spatial lightmodulator 48 may include a single 2D image. As all the illuminatorelements 15 n may simultaneously be operated, the sensor system might beunused or omitted. It would be further desirable to provide the powersaving advantages of increased display gain in autostereoscopic mode foroff-axis viewing positions.

FIGS. 15E to 15F are schematic diagrams illustrating a graph of luminousflux distribution for an autostereoscopic Lambertian display system. Inthis case, the scaled luminous fluxes vary across the array ofilluminator elements 15 n in a luminous flux distribution that is fixedwith respect the position in the lateral direction, but the displaydevice is operated to provide an autostereoscopic 3D display asdescribed above by means of the SLM 48 displaying temporally multiplexedleft and right eye images and light being directed into viewing windowsin positions corresponding to the left and right eyes of an observersynchronously with the display of left and right eye images.

FIG. 15E illustrates an example in which a constant luminous fluxdistribution 269 is used to provide a luminous intensity distribution266 that is Lambertian as shown in FIG. 14B. Viewing windows 26 may beformed from illumination of illuminator elements in a sub-array 520 fora left eye phase and a sub-array 522 in a right eye phase. As the eyesof observer 99 move off-axis, then the scaled luminous fluxes may have aconstant value and the display may maintain a Lambertian appearance withviewing angle. Advantageously the number of illuminator elementsilluminated is much reduced compared to the arrangement of FIG. 15D. Theluminance for each eye will be substantially constant, reducing theappearance of depth errors in autostereoscopic viewing and increasingviewer comfort.

FIGS. 15F and 15G illustrate an example in which a non-linear luminousflux distribution 273 is used to provide a luminous intensitydistribution 272 having a gain greater than one. Thus the illuminatorelements may be controlled to output light with scaled luminous fluxes263 that vary across the illuminator elements in dependence on thedetected position of the observer 99. For example FIG. 15F illustratesthe case that the detected position of the observer is aligned with theoptical axis and FIG. 15F illustrates the case that the detectedposition of the observer is offset. Sub-arrays 520, 522 of illuminatorelements for the left and right eyes are selectively operated, changingwith the detected position of the observer. The sub-arrays 520, 522 ofilluminator elements have varying luminous intensity across their widthto achieve the gain. Thus the illuminator elements may be controlled tooutput light with scaled luminous fluxes 263 that vary across theilluminator elements in dependence on the detected position of theobserver 99 in a manner that produces a luminous intensity 264 of theoutput light that varies with the angle 262 of the output light in theluminous intensity distribution 272 having a gain greater than one.Advantageously, the power consumption of the array 15 of illuminatorelements is reduced compared to the arrangement of FIG. 15E, since notall the illuminator elements are operated at any one time.

Advantageously off-axis power consumption may be reduced compared toon-axis positions, reducing power consumption further.

FIG. 15H is a schematic diagram illustrating a further graph of luminousflux distribution for an autostereoscopic display system with a gain ofgreater than one and an off-axis viewing position. In this embodiment,the arrays 520, 522 of illuminator elements are arranged to track thedistribution 273 shown by matching the scaled luminous fluxes atposition 524, but have equal luminous flux linear density across thesub-arrays 520 and 522. Thus, the scaled luminous fluxes vary across thearray of illuminator elements 15 n in a luminous flux distribution thatis not fixed with respect the position in the lateral direction theilluminator elements but is varies in dependence on the detectedposition of the observer. In particular, this is controlled in a mannerthat produces a luminous intensity 264 of the output light that varieswith the angle 262 of the detected position of the observer in theluminous intensity distribution 272 having a gain greater than one.

Thus, the left and right eye viewing windows may be arranged withluminous intensity 264 distributions to achieve substantially equalluminance for each eye in the manner of Lambertian distribution ofluminous intensity. Again, the power consumption of the array 15 ofilluminator elements is reduced compared to the arrangement of FIG. 15E,since not all the illuminator elements are operated at any one time.Off-axis power consumption may be reduced compared to on-axis positionsand each eye may perceive an image luminance that is substantiallyequally, reducing depth errors in autostereoscopic viewing andincreasing viewer comfort.

FIG. 16 is a schematic diagram illustrating an illuminator elementaddressing apparatus. Thus illumination controller 74 may be arranged toaddress illuminator elements 243 of array 15 by means of drive lines 244to provide drive signals to the array of illuminator elements. Atposition 261 across the array 15, illuminator elements 243 may havevarying scaled luminous fluxes, as described elsewhere. In operation,scaled luminous flux variations may be achieved by means of controllingcurrent into the respective sources 243 by the illumination controller74 and control system. Control of luminous flux distribution may beachieved by means of current control or voltage control for example.Further the variation may be adjusted according to user requirements,thus a user may select a high gain, low power consumption mode or mayselect a wide field of view mode.

In the present embodiments, the array 15 of illuminator elements may bearranged at the input end 2 of a waveguide 1, for example as shown inFIG. 13 or may be at the input end 1103 of a wedge waveguide 1104 asshown in FIG. 12.

FIG. 17 is a schematic diagram illustrating a graph of luminousintensity for an array of optical windows and respective opticalwindows, in a further example in which the display device is operated toprovide an autostereoscopic 3D display as described above, and thescaled luminous fluxes vary across the array of illuminator elements 15n in dependence on the detected position of the observer. Thusilluminator elements of array 15 may provide a constant scaled luminousfluxes and an optical system in such conditions may achieve a Lambertianprofile of luminous intensity at the window plane. Each source of array15 may provide an optical window 249 in the window plane 106. A left eyeviewing window 247 is provided by an array 530 of left eye opticalwindows and a right eye viewing widow 251 is provided by an array 532 ofright eye optical windows, in left and right eye illumination phasesrespectively. The left and right eye viewing windows 247 and 251 aregenerated in dependence on the detected position of the observer. Thescaled luminous fluxes vary across the array of illuminator elements 15n in accordance with a luminous flux distribution that produces aluminous intensity of the output light in the optical windows thatvaries with the angle of the output directions in a luminous intensitydistribution 267. This produces a luminous intensity of the output lightin the viewing windows 247, 251, on addition of optical window arrays530, 532, that varies with the angle of the detected position of theobserver in the lateral direction in a luminous intensity distribution266. Thus, the display device 100 may be achieved with substantiallyLambertian output appearance. Thus luminous intensity distributions 266,267 may be Lambertian. In embodiments described herein, similarformation of viewing windows from optical windows are provided.Different luminous intensity distributions for viewing windows 26 otherthan Lambertian are further provided by combining arrays of opticalwindows 249.

FIG. 18 is a schematic diagram illustrating a graph of optical windowluminous intensity 264 against viewing position 262 for a waveguide 1including uniform luminous intensity of illuminator elements in array15, in an alternative construction of the waveguide 1. The embodimentsdescribed above assume that for a luminous flux distribution 269, asubstantially Lambertian luminous intensity distribution 266 isachieved. In embodiments of the waveguide 1, non-Lambertian behaviourmay be exhibited by the waveguide 1 as will be described with referenceto FIGS. 19A-19B. More specifically, a luminous intensity distribution268 may be achieved by the waveguide 1 including horn shaped features540, 542. Thus for a matched on-axis luminous intensity 507, theoff-axis luminous intensity is substantially higher than that providedby a Lambertian output luminous emittance distribution 266; in otherwords the luminance of the display may increase for some off-axisviewing positions. It would be desirable to reduce power consumption ofthe array 15 of illuminator elements when operating in 2D mode by meansof compensation of the distribution 268.

FIG. 19A is a schematic diagram illustrating a perspective view of alight ray incident with a first direction onto a light extractionfeature of a waveguide 1 of an alternative construction in which thelight extraction feature 12 are not mirrored and instead reflect lightby TIR. Surface 8 of the waveguide 1 may include intermediate regions 10and light extraction features 12, which may be referred to herein aslight extraction facets. On axis light rays 300 at an angle 307 to thenormal 304 to feature 12 and that are parallel to features 10 arereflected by total internal reflection at feature 12 along ray 302.However, light rays 301 that remain in the same x-z plane as rays 300that at a smaller angle 307 to the surface normal 304 of features 12 maybe transmitted to ray 305. Thus rays 305 may be lost to the outputluminous intensity distribution.

FIG. 19B is a schematic diagram illustrating a perspective view of alight ray incident with a second direction onto a light extractionfeature of a waveguide 1 of the same construction as FIG. 19A. Off-axisrays 308 may be provided by an off-axis illuminator element of array 15and are incident at an angle 308 to on-axis rays in the x-y plane. Rays309 that for on axis incidence that would be transmitted by the feature12 are incident at an angle 312 to the normal 304 that has a greaterresolved angle than angle 307 and thus undergoes total internalreflection at the feature 12. Thus light rays 311 are reflected ratherthan transmitted. This means that the luminous flux of the output lightreflected by the feature 12, as a proportion of the luminous flux of theinput light, varies for different illuminator elements 15 n. Thiscontributes to increased luminous intensity 264 at the respectiveoff-axis position 262 in the window plane, and produce the horn features540, 542. The control system is arranged to control the illuminatorelements to output light with scaled luminous fluxes that vary acrossthe array of illuminator elements in manner that compensates for thisvariation in the luminous flux of the output light reflected by thefacets, as follows.

FIG. 20A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide and a method to adjust scaled luminous fluxes of theilluminator elements 15 n. In order to achieve a luminous intensitydistribution 266 that is Lambertian, the luminous intensity distributionmay be modified as shown by arrows 270. This may be achieved bycontrolling the scaled luminous fluxes of the illuminator elements 15 nto vary across the array 15 in luminous flux distribution as follows.

FIG. 20B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of illuminator elements and method to adjustthe scaled luminous fluxes of the illuminator elements 15 n for awaveguide 1. Arrows 271 shows the drop in the scaled luminous fluxes,compared to constant luminous flux distribution 269, for respectivelight sources corresponding to arrow 270 at respective optical windowposition 262, to reach a non-linear luminous flux distribution 277 thatprovides the luminous intensity distribution 266 that is Lambertian.Thus arrows 271 that may have a length proportionately equivalent to thelength of arrows 270 for a respective equivalent positions 262, 261, andthe luminous flux distribution 277 may be provided. Thus, the luminousflux distribution 277 has a global maximum 508 of luminous flux andreduces on either side of the global maximum 508 is provided across thearray 15. The global maximum 508 occurs in respect of the illuminatorelements aligned with the optical axis of the waveguide 1. The luminousflux distribution 277 also has ‘anti-horn’ features 544, 546 tocompensate for horn features in the distribution 268.

Further in combination with the observer tracking arrangement of FIG.13, the illuminator elements of the array 15 corresponding to opticalwindows that are not seen by the observer 99 may be extinguished,advantageously reducing power consumption Thus the illuminator elementsmay be controlled to output light with scaled luminous fluxes 263 thatvary across the illuminator elements in dependence on the detectedposition of the observer 99 in a manner that produces a luminousintensity 264 of the output light that varies with the angle 262 of thedetected position of the observer 99 in a luminous intensitydistribution 266 that is Lambertian.

Advantageously, the non-Lambertian luminous intensity output of thewaveguide 1 can be compensated for, achieving substantially uniformluminance across the viewing angle of the display.

FIG. 21A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide and a method to adjust the scaled luminous fluxes of theilluminator elements 15 n. In another embodiment, the luminous intensitydistribution 268 can be adjusted to achieve luminance intensitydistribution 272 having a gain greater than one. In particular, theluminous intensity distribution may be modified as shown by arrows 270.This may be achieved by controlling the scaled luminous flux of theilluminator elements to vary across the array 15 in luminous fluxdistribution as follows.

FIG. 21B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of illuminator elements and method to adjustthe scaled luminous fluxes of the of the illuminator elements 15 n for awaveguide 1. Thus the luminous flux distribution 275 may be achieved ina similar manner to that described for FIG. 20B. Arrows 271 shows thedrop in scaled luminous flux, compared to constant luminous fluxdistribution 269, for respective light sources corresponding to arrow270 at respective optical window position 262, to reach a non-linearluminous flux distribution 275 that provides the luminous intensitydistribution 272 having a gain greater than one. Thus, the luminous fluxdistribution 275 has a global maximum 508 of scaled luminous flux andreduces on either side of the global maximum 508 is provided across thearray 15. The global maximum 508 occurs in respect of the illuminatorelements aligned with the optical axis of the waveguide 1. The luminousflux distribution 277 also has ‘anti-horn’ features 544, 546 tocompensate for horn features in the distribution 268.

Further, the array 15 may include a luminous flux distribution for aconstant drive current that varies as shown by distribution 550. Such avariation can be removed during the correction of scaled luminous fluxesdescribed by non-linear luminous flux distribution 275 by knowledge ofthe distribution 550 prior to applying correction shown by arrows 271.Thus the total luminous intensity under curve 272 may be the same as thetotal luminous intensity under curve 266.

In one embodiment, illuminator elements of the array 15 may becontrolled so that their scaled luminous fluxes vary in accordance withthe non-linear luminous flux distribution 273, thereby providing thedisplay device 100 with the luminous intensity distribution 272 having again greater than one. That is, the luminous intensity 264 of the outputlight varies with the angle 262 of the output directions in an actualluminous intensity distribution 272 that is greater than a notionalluminous intensity distribution 266 that is Lambertian and has the sametotal luminous intensity over all output directions as the actualluminous intensity distribution 272.

In an illustrative embodiment, a 15.6″ spatial light modulator may beilluminated by a waveguide 1 including an array 15 of 86 illuminatorelements, each that may achieve 16 lumen per steradian of optical outputin CW mode in a Lambertian distribution for the illuminator elementarranged in air. Such illuminator elements may each be driven with 350mW of electrical power if the luminous flux distribution 269 isprovided, delivering a total array power consumption of 30 W. For thesame on-axis luminous intensity (and thus luminance), and applying thedistribution 272 to luminous intensity, and thus a luminous fluxdistribution similar to distribution 275, the total electrical powerconsumption of the array 15 may be reduced to 16 W. In autostereoscopicmode of operation, the illuminator elements are operated in pulsed mode,for example with a 25% duty cycle and 50% current overdrive, thus theilluminator elements may have substantially a 50% total luminancecompared to CW mode. By matching the 2D and 3D on-axis luminousintensities, the total power consumption of the array 15 may be reducedto 8 W.

FIG. 22 is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane,corresponding to the angle of the output light in the lateral direction,in an example in which the display device is operated to provide anautostereoscopic 3D display as described above and a method to adjustscaled luminous fluxes of illuminator elements 15 n for left and righteye illumination phases.

In FIG. 22, the luminous intensity distribution 272 for a gain ofgreater than one is marked, and left eye 560 and right eye 562 positionsare marked for observer 99 at various viewing positions. For an observer99 moving to the right of the optical axis, the left eye position isindicated to follow the luminous intensity distribution 272. For anequal display luminance, the display should achieve a difference in theluminous intensity between the left and right eyes of the observer thatis Lambertian. Thus Lambertian distribution 266 may be arranged to passthrough the left eye 560 and provide a desired luminous intensity 564 atthe right eye position 562 across the window plane 106 position 262.Thus points 564 can be interpolated across the window plane for theright eye to provide right eye luminous intensity distribution 280. Foran observer 99 moving to the left of the optical axis, the right eye mayfollow the luminous intensity distribution 272, while the left eye mayprovide distribution 282 in a similar manner.

FIG. 23A is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide 1 and a method to adjust scaled luminous fluxes of illuminatorelements 15 n for the right eye illumination phase. Thus, aftercorrection as shown by the arrows, for the right eye 562, the waveguideluminous intensity distribution 268 may be modified to the luminousintensity distribution 272 for positions to the left of the optical axisand distribution 280 to the right of the optical axis. Similarly for theleft eye 560, the waveguide luminous intensity distribution 268 may bemodified to the luminous intensity distribution 272 for positions to theright of the optical axis and distribution 282 to the left of theoptical axis. This may be achieved by controlling the scaled luminousflux of the illuminator elements 15 n to vary across the array 15 independence on the detected position of the observer and in dependence onwhether the output light is modulated by a left or right image, asfollows as follows.

FIG. 23B is a schematic diagram illustrating a graph of luminous fluxdistribution for an array of illuminator elements for left and right eyeillumination phases and method to adjust scaled luminous fluxes of theilluminator elements 15 n for a waveguide 1 for the right eyeillumination phase. Thus, the right eye luminous flux distribution 279may be different to the left eye luminous flux distribution 281. Arrows271 shows the drop in scaled luminous flux, compared to constantluminous flux distribution 269, for respective light sourcescorresponding to arrow 270 at respective optical window positions 262,to reach the right eye luminous flux distribution 279 when the outputlight is modulated by a right image and the left eye luminous fluxdistribution 281 the output light is modulated by a left image.

Advantageously, the power consumption of the array 15 may besubstantially reduced in a 2D mode while maintaining equal luminancebetween left and right eyes, thus improving display luminance.Similarly, an observer tracking display for the purposes of anautostereoscopic display, privacy display or high efficiency 2D modedisplay can achieve low power consumption while maintaining comfortableluminance properties of the display for left and right eye images.

In the above examples, control of the scaled luminous flux is effectedto provide a luminous intensity distribution 266 that is Lambertian or aluminous intensity distribution 272 that has a gain greater than one.However, this is not limitative and the scaled luminous flux may becontrolled so to provide a luminous intensity distribution of othershape. Some examples are now given.

FIG. 24 is a schematic diagram illustrating a graph of optical windowluminous intensity against viewing position in the window plane of awaveguide 1, corresponding to the angle of the output direction. It maybe desirable to further modify the luminance intensity distribution, forexample to provide a broader region of Lambertian behaviour near toon-axis positions but to increase the slope of the distribution 274 inoff-axis positions in comparison to distribution 272. Advantageously thedisplay may have substantially Lambertian behaviour on-axis, may havesufficient light for off-axis viewing while achieving low powerconsumption for off-axis viewing.

FIG. 25 is a schematic diagram illustrating a further graph of opticalwindow luminous intensity against viewing position in the window planeof a waveguide 1, corresponding to the angle of the output direction.Thus distribution 276 may have a very narrow central region and a fastdrop-off to a background illumination profile.

The distributions for example 266, 272, 274 or 276 may be controlled bya user selection or by automatic selection by a control system, forexample by means of luminous flux controller 580 shown in FIG. 13.Advantageously the properties of the display may be modified to suitremaining battery lifetime, privacy requirements, multiple viewers,display brightness environment, user experience and other userrequirements.

FIG. 26A is a schematic diagram illustrating a further graph of opticalwindow luminous intensity against viewing position in the window planeof a waveguide 1, corresponding to the angle of the output direction,and FIG. 26B is a schematic diagram illustrating a graph of luminousflux distribution for an array of illuminator elements to compensate forlight source degradation. Illuminator elements such as LEDs includinggallium nitride blue emitters and yellow phosphors may undergo ageingwherein the scaled luminous fluxes and chromaticity may vary with usage.In particular, on-axis illuminator elements may be used more frequentlythan off-axis illuminator elements that may provide a non-uniformdegradation in luminous emittance. Such errors can be corrected as shownby distribution 279 of scaled luminous fluxes from the respectiveilluminator elements. Advantageously, the luminance distribution of thedisplay may be maintained throughout the device lifetime.

FIG. 27A is a schematic diagram illustrating a front view of adirectional display apparatus in landscape mode. Thus display 290 mayprovide vertical viewing windows 292. FIG. 27B is a schematic diagramillustrating a front view of a directional display apparatus in portraitmode. Thus when display 290 is rotated, the viewing windows 292 becomehorizontal when such a display is used in privacy mode or green mode, itmay be desirable to adjust the luminous intensity distribution as shownin FIG. 27C. FIG. 27C is a schematic diagram illustrating a furthergraph of optical window luminous intensity against viewing position inthe window plane of a waveguide 1 for the arrangement of FIG. 27B. Thusdistribution 294 may be offset from the on-axis position, achieving highbrightness for the preferred vertical viewing position, while achievinglow power consumption for other viewing angles, while maintaining adisplay that is visible.

FIG. 28 is a schematic diagram illustrating a side view of a waveguide 1and viewing windows 26 for observer movement in a directionperpendicular to the lateral direction at the same distance from thewaveguide 1 (hereinafter referred to as “vertical”) and/or along thenormal to the first guide surface of the waveguide 1. FIG. 29 is aschematic diagram illustrating a graph of luminous flux distribution foran array 15 of illuminator elements and method to adjust the scaledluminous fluxes of the illuminator elements 15 n for a waveguide 1 withthe observer movement of FIG. 28. In this case the scaled luminousfluxes are controlled in dependence on the position of the observer, asdetected by the sensor system as described above, to provide aluminousintensity of the output light that varies with the angle of the detectedposition in the observer in the lateral direction, and also varies withthe vertical position of the observer and/or the position of theobserver along the normal to the first guide surface of the waveguide 1.In a first embodiment, for vertical viewing positions 601, 603, 605, therespective luminous flux distributions 600, 602, 604 may be provided.Advantageously, as the observer moves away from a preferred verticalviewing position, the output luminous flux distributions may be adjustedaccordingly. The profiles of the distributions 600, 602, 604 are shownhere as being different shapes; the shape of the distributions can bemodified for example to achieve a high quality Lambertian output forpreferred vertical viewing angles, but higher gain performance fordifferent viewing angles, thus saving power for viewing from thosedirections. Similarly, viewer movement to plane 607 away from the windowplane 106 may be used to modify the luminous flux distribution acrossthe array 15.

Observer tracking displays may suffer from image flicker for movingobservers. For an observer approximately at the window plane, the wholedisplay may change intensity simultaneously due to non-uniformities ofthe integrated optical window array 121 in the window plane. If theobserver is away from the window plane or the illumination system outputis aberrated, then different regions of the display surface may appearto flicker by different amounts. It is desirable that the tracking andillumination steering systems cooperate to remove flicker for movingobservers, which may be achieved by reducing the amplitude of theperceived intensity variation.

For example, an eye that moves into a non-illuminated optical window maysee a large intensity variation on the display. It may be desirable toincrease the intensity of this window prior to movement so that theflicker artefact may be reduced. However, increasing the intensity ofoptical windows, particularly the interocular optical windows locatedbetween the observer's eyes may increase image cross talk, and maydegrade 3D image quality. Further increasing window intensity at timeswhen the observer is substantially stationary, when low cross talk isdesirable, and when the observer is moving, when low flicker isdesirable, may result in an additional flicker artefact.

FIG. 30 is a schematic diagram illustrating an arrangement of viewingwindows during observer motion between viewing positions in an examplein which the display device is operated to provide an autostereoscopic3D display as described above. Further, FIG. 30 shows another switchingarrangement for an array 721 of optical windows during movement of anobserver and arranged to reduce display flicker while substantiallymaintaining reduced image cross talk.

In FIG. 30, the illuminator elements 15 n are controlled to direct lightinto a left viewing window 730 and a right viewing window 732 that eachcomprise plural optical windows, in positions corresponding to left andright eyes of an observer, in dependence on the detected position of theobserver. In this example, there is a gap of one optical window betweenthe left and right viewing windows 730 and 732, but alternatively theremay be no gap or a larger gap. In this example, each of the left andright viewing windows 730 and 732 comprises five optical windows, but ingeneral a viewing window could comprise any number of optical windows.

In the case that the viewing windows comprise at least two opticalwindows, then the illuminator elements 15 n may be controlled to outputlight with a scaled luminous fluxes that varies across each of the leftand right viewing windows 730 and 732. In the example of FIG. 30, thedistribution of the scaled luminous fluxes across each of the left andright viewing windows 730 and 732 is as follows.

The distribution has a global maximum for the optical windows 730 and734 which are central in the left and right viewing windows 730 and 732.In addition, the distribution for the the left and right viewing windows730 reduce on both sides of that global maximum, that is in opticalwindows 722 and 726 of the left viewing window 730 and in opticalwindows 724 and 728 of the right viewing window 732. The example of FIG.30 shows a single optical window 722, 724, 726, 728 on each side ofoptical window array 730 having a reduced, scaled luminous flux, but ingeneral more than one optical window may have a reduced, scaled luminousflux of the same or different levels. The reduced, scaled luminous fluxof optical window 722, 724, 726, 728 may be achieved by change in theillumination level, pulse width or pulse pattern, or any combination ofthese.

Advantageously using the luminous flux switching of FIG. 30 may reducethe perception of flicker by the observer while substantiallymaintaining low cross talk and thereby may improve display quality. Suchan embodiment may achieve windows suitable for both stationary andmoving observers, while reducing the effect of flicker from increasingthe intensity of adjacent or interocular optical windows. Flicker isreduced in particular by reducing the scaled luminous flux on the sideof the viewing window adjacent the other viewing window, i.e. for theleft viewing window 730 in the optical window 726 on the side of theright viewing window 732, and similarly for the right viewing window 732reduces below the global maximum in the optical window 724 on the sideof the left viewing window 730.

Advantageously the number of optical windows illuminated may be reduced,for example, in battery powered equipment which may extend batteryoperation time. Reducing the number of illuminated optical windows mayincrease the perceived flicker.

FIGS. 31 to 35 illustrate some other distributions of scaled luminousflux that may be applied across the left and right viewing windows atthe window plane, being schematic graphs of luminous intensity 700against input position 702 in the lateral direction (y-direction).

FIG. 31 shows distributions 708, 710 for left eye 110 and right eye 108of an observer i. Eye positions 704, 706 can then be used to determinethe intensity and cross talk at a given location across the windowplane. The arrangement of FIG. 31 would be for near-perfect windows,such that no cross talk is observed, however such windows are nottypically present.

FIG. 32 shows distributions 712, 714 having sloped sides so that eyessee some light from adjacent viewing windows, creating undesirable imagecross talk and visual fatigue for users.

FIG. 33 shows distributions 712, 714 which are similar to FIG. 32 butmore widely separated, for example as would be achieved by illuminationwherein the illuminator elements to achieve left and right viewingwindows 730, 732 having substantially the same scaled luminous fluxes.Advantageously the cross talk is reduced. However, small movements ofthe observer can reduce display intensity that creates flicker for amoving observer in observer tracked displays.

FIG. 34 shows distributions 716, 718 similar to that which would beachieved in the arrangement of FIG. 30. Such an arrangement increasesthe intensity of the windows near the observer's nose for the movingobserver, but has reduced image cross talk for a stationary observer.

FIG. 35 shows the distribution 714 for the right eye illustratingnon-uniformities of display uniformity arising from a non-uniform windowintensity distribution. The right eye 108 of the observer may bepositioned at a distance 746 between the window plane 106 and displaydevice 720. Thus light rays that are directed from the display region742 to the eye 108 are those rays that would have been directed to theuniform portion 723 of the window 714 and light rays from display region724 are those rays that would have been directed to non-uniform portion725 of the window 714. In this manner, non-uniform window structure canresult in display non-uniformities for observers not in the window planeso that regions 742 and 744 of display 720 have different intensityprofiles. It is thus desirable to reduce ‘hard’ (sharp gradient) windowboundaries to reduce visibility of artefacts in the display plane. Thusthe grey scale arrangement of FIG. 30 can advantageously achieveimproved performance of uniformity visibility for moving and non-movingobservers, while achieving reduced image cross talk.

FIGS. 36-41 are schematic diagrams illustrating some further non-uniformluminous flux distributions that may be applied across the left andright viewing windows at the window plane, being schematic graphs ofscaled luminous flux 263 against input position 261 in the lateraldirection (y-direction). In each case, the left eye viewing window 730comprises plural optical windows 770 with luminous flux distribution 772and the right eye viewing window 732 comprises plural optical windows771 with luminous flux distribution 774. In each case, within each ofthe left and right viewing windows 730, 732, the luminous fluxdistributions 772, 774 are non-uniform with global maxima 773, 775respectively, and reduce on both sides of those global maxima 773, 775.In these examples, each of the left and right viewing windows 730 and732 comprises five optical windows, but in general similar distributionsreducing on both sides of a global maximum could be provided to viewingwindows comprising any number of at least three optical windows.

The global maxima 773, 775 of the illuminator elements may be directedby the optical system towards the pupils of an observer in an observertracked display in dependence on the detected position of the observer99, obtained using the sensor system. The global maxima 773, 775 may forexample be arranged to be positioned 32 mm each side of the measurednose position of an observer 99.

It may be seen that the luminous flux distributions of FIG. 36substantially achieve similar profiles of optical and viewing windowarrangements, however diffusion and scatter within the optical systemmay serve to blur such window arrangements in comparison with thedistributions of FIG. 36.

In comparison with viewing windows with substantially constant luminousflux distributions, the brightness of the observed image may beobtained, particularly in the central region of the display area. Theamount of light present in the system may be reduced, and thus crosstalk from stray light may be minimised. The visibility of flickerartefacts for a moving observer may be further reduced, by providingwider total window width for similar or less cross talk. Further, thetotal power consumption of the apparatus may be reduced, increasingefficiency and reducing cost.

Whereas FIG. 36 illustrates the situation that the detected position ofthe observer 99 is aligned with the optical axis 232 of the waveguide 1,FIG. 37 illustrates the situation for an observer 99 that has movedlaterally with respect to the optical axis 232 of the waveguide 1. Thus,the luminous flux distributions 772, 774 of the left and right viewingwindows 730 and 732 may be displaced laterally in dependence on thedetected position of the observer 99, while maintaining variations inintensity across the viewing windows. Advantageously, the change inintensity of the outer optical windows can be reduced in comparison touniform scaled luminous flux illuminator elements for a viewing window,reducing flicker for a moving observer 99.

FIG. 38 illustrates an alternative form of the left and right viewingwindows 730 and 732 for an observer that has moved laterally withrespect to the optical axis 232 of the waveguide 1. In this case, theglobal maxima 773, 775 may be arranged to follow luminous fluxdistributions 777, 779 to achieve a Lambertian luminous intensitydistribution of optical windows, and may track luminous intensitydistribution 769 for example. Illuminator elements 770, 771 that are notat the global maxima 773, 775 may be modified in scaled luminous fluxaccordingly. Advantageously the display may appear equally bright from arange of viewing angles, while achieving reduced cross talk, flicker andpower consumption. Alternatively, further power saving characteristicscan be achieved by further reducing display luminance for off-axisviewing positions. The distribution 769 may be modified to increase thegain of the display and further reduce power consumption for off-axispositions.

FIG. 39 illustrates an alternative form of the left and right viewingwindows 730 and 732 for an observer that, in comparison to FIG. 36, hasmoved longitudinally along the optical axis 232 of the waveguide 1, thatis along the normal to the first guide surface of the waveguide 1. Inthis case, the scaled luminous flux is also varied in dependence on thedetected longitudinal position, in this example to provide respectiveoptical windows and viewing windows that comprise flatter luminous fluxdistributions. As shown in FIG. 35, the variation of luminous intensityacross the optical windows can provide a variation in displayuniformity. As an observer moves longitudinally with respect to thedisplay, non-uniformities may increase as more optical windows may becaptured across the display area. Thus it may be desirable to reduce theintensity variation across the optical windows.

Further, a user may select a preferred gain profile for the wholedisplay, and within windows to suit personal preferences for efficiency,cross talk, uniformity and image flicker as well as measured eyeseparation.

FIGS. 40 and 41 illustrate alternative forms of the left and rightviewing windows 730 and 732. Herein, the scaled luminous flux ofilluminator elements that are directed to optical windows between theobserver's eyes, and are thus located between the global maxima 773,775, is reduced less than the reduction of scaled luminous flux of theilluminator elements that are located outside the maxima 773, 775. Suchan arrangement provides some light towards the edge of the display forobservers 99 away from the window plane 106, while achieving relativelyuniform brightness for regions in the centre of the display.Advantageously display uniformity is maintained while perceived imageflicker is reduced.

Whereas in the above examples the display device is operated to providean autostereoscopic 3D display, similar variation in the scaled luminousflux across a viewing window may be applied in the case that displaydevice is operated to display a 2D image and the control system operatethe light sources 15 to direct light into a single viewing window independence on the detected position of the observer 99. An example ofthis will now be described.

FIG. 42 is a schematic diagram illustrating a 2D directional display inlandscape orientation. The display device 100 is arranged to provide asingle viewing window 790 in dependence on the detected position of theobserver 99 in the lateral direction. The light sources are controlledto output light with scaled luminous fluxes that vary across the arrayof light sources to provide a luminous intensity that varies with theangle of the detected position of the observer in the luminous intensitydistribution 793 with global maxima 773, 775 substantially aligned withright and left eyes 791, 792 of observer 99. The array 15 is arrangedwith non-illuminated illuminator elements 797 and illuminated group 804of illuminator elements of array 15 achieving the global maxima 773, 775in the viewing window 790. The array 15 may be arranged with asubstantially constant pitch 803 to minimise non-uniformities in theviewing window when imaged in combination with asymmetric diffuser 68and other scattering components in the display.

Thus the array 15 of illuminator elements may be disposed at differentinput positions 261 with a constant pitch 803 in the lateral directionacross the input end of the waveguide, whereby the scaled luminousfluxes are the actual luminous fluxes of the illuminator elements 15 n.

Further illuminator elements 802 may be provided corresponding to offaxis viewing positions that have a larger pitch and/or lower scaledluminous flux than the illuminator elements for on axis viewingpositions. In combination with the control of scaled luminous flux ofthe illuminator elements for more on-axis positions, such illuminatorelements can reduce cost of the array of illuminator elements withoutcompromising the desired optical window output.

FIG. 43 is a schematic diagram illustrating a graph of scaled luminousflux against input position for the display of FIG. 42. Thus theillumination of a 2D display may be arranged to provide reduced powerconsumption compared to a Lambertian illuminated display. The operationof the display is similar to that shown for FIGS. 36-41, however, thegroup 804 is arranged to be illuminated in a single phase for both eyesand may provide continuous operation. Advantageously the powerconsumption of the device may be reduced, while achieving substantiallythe same brightness of the centre of the display for an observer at thewindow plane, and minimizing display flicker for a moving observer.Further, the spatial light modulator 48 may operate continuously ratherthan in synchronization with left and right eye illumination phases,reducing cost of the spatial light modulator 48.

FIG. 44 is a schematic diagram illustrating a 2D directional display inportrait orientation. The height 808 of the window 795 (in the lateraldirection) in the portrait orientation is reduced compared to the width806 of the window (in the lateral direction) in the landscapeorientation as the eyes are arranged parallel to the extent in thex-axis of the viewing windows 795 and luminous intensity distribution796 may be provided. Thus the power consumption can advantageously befurther reduced compared to the arrangement of FIG. 42.

FIG. 45 is a schematic diagram illustrating a graph of scaled luminousflux against input position for the display of FIG. 44. Thus, theluminous flux distribution 800 of optical windows 812 may be of narrowerwidth than the arrangement of FIG. 44 to form viewing window 794 and asingle maximum 810 may be provided so that advantageously the centre ofthe display is optimally illuminated for on-axis viewing positions awayfrom the window plane. The viewing window 794 may be stationary or maybe adjusted in correspondence to the detected position of observer 99.

Thus a step of selectively operating the illuminator elements to directlight into varying optical windows corresponding to said outputdirections may be performed to direct light into at least one viewingwindow 794 comprising at least two simultaneously illuminated opticalwindows 812, the illuminator elements being controlled to output lightwith luminous flux linear density 263 that varies in dependence on thedetected position of the observer 99 and further varies across theplural optical windows 812 of said at least one viewing window 794.

FIG. 46 is a schematic diagram illustrating a control system and frontview of a directional backlight apparatus comprising a directionalbacklight as described above including a waveguide 1 and array 15 ofilluminator elements. The directional backlight apparatus includes acontrol system, as described above, that implements a method ofcontrolling the illuminator elements 15 n making a calibration of thedrive signals, as follows.

Light rays 210 from illuminator element 216 are directed to reflectiveend 4, reflected and directed back towards the input end 2. Some of thelight from source 216 will be extracted by means of light extractionfeatures 12, while some of the light will be incident on at least aportion of the input end 2. Sensor elements 208, 214 may be arranged atthe input end in regions 209, 215 outside the lateral extent of thearray 15 on both sides of the array 15. In regions 212, an illuminationvoid is present so that light from source 216 will not be substantiallyincident on sensor 214; however light rays from source 216 will beincident on sensor 208. Each sensor 208, 214 may include a lightintensity measurement sensor. Preferably as shown in FIG. 46, thesensors 208, 214 may include optical filters 202, 206 and lightintensity sensors 200, 204. Such an arrangement may advantageouslyprovide a measurement of both the light intensity and a measurement ofchromaticity coordinate for the light from source 216. In a similarmanner, light rays 220 from source 218 may not be incident on sensor208, but will incident on sensor 214. For on-axis measurement, sensors208, 214 may both detect light from respective on-axis illuminatorelements 217.

Measured signals from sensors 208, 214 may be passed to illuminationcontroller 74 which drives illuminator elements of array 15 using anilluminator element driver 233 which may be a current driver with greylevel control to drive lines 244 to provide drive signals to the arrayof illuminator elements. The illumination controller 74 calibrates thedrive signals supplied to the illuminator elements 15 n in response themeasured signals representing the sensed light, as follows.

Array luminous flux distribution controller 224 may include for examplea stored reference grey level profile 230 from front of screenmeasurements that may be provided at the time of manufacture, forexample including luminous flux distribution data for distribution 550in FIG. 21B. This allows the control system to output scaled luminousfluxes that have a predetermined distribution across the array of lightsources, for example to vary the scaled luminous fluxes as describedabove.

Data from sensors 208, 214 may be supplied for example to calibrationmeasurement system 222 that may provide data to a look up table 226within the luminous flux distribution controller 224. Further selectionof luminous intensity distribution (for example to select betweenluminous intensity distributions 266, 272, 274, 276, 294) may beprovided by selection controller 228. Selection controller may have userinput or an automatic input that is determined by sensing of displayviewing conditions. For example the number of viewers, the roombrightness, display orientation, the image quality settings and/or thepower savings mode settings may be used to vary the selecteddistribution.

In device manufacture, the output of the sensors 208, 214 in response toeach of the light sources of the array 15 may be compared to the signalfrom a camera or detector placed in the window plane of the display.This achieves an initial calibration or referencing of the internalsensors with respect to light in the window plane. Such calibration maybe stored in a look up table or similar.

In operation of a calibration mode, a single illuminator element of thearray 15 is illuminated and sensors 208, 214 may measure a signal forthe said illuminator element. The said illuminator element isextinguished and the next source of the array operated and a measurementtaken. The output of the array of measurements is compared with afactory calibration so that the output luminous intensity for the givenluminous flux distribution can be interpolated. The appropriate luminousflux distribution for the required luminous intensity distribution isthen derived by the controller 224 and the illuminator elementcontroller 233 appropriately configured to achieve the desired luminousflux distribution.

Advantageously the light from the whole array 15 may be measured by acombination of sensors 208, 214 and a desired luminous intensitydistribution may be achieved.

Thus said sensing of light incident on the input end 2 may use sensorelements 208 arranged at region 209 of the input end 2 outside the array15 of illuminator elements in the lateral direction. Said sensing oflight incident on the input end 2 may use sensor elements 208, 214arranged at regions 209, 215 of the input end 2 outside the array 15 ofilluminator elements in the lateral direction on both sides of the arrayof illuminator elements.

The sensor system may be arranged with the waveguide 1 only during thefabrication of the display for characterization purposes and removedafter completion of product fabrication. Preferably the sensor systemmay be arranged with the waveguide 1 during normal operation. Thein-field calibration phase may be applied during display switch-on. Thespatial light modulator may be arranged with a black image duringcalibration to remove visibility to the user of the calibration phase.The calibration phase may be repeated on a daily, weekly or monthlybasis for example to compensated for ageing artefacts as shown in FIGS.26A-B.

FIG. 47 is a schematic diagram illustrating a control system and frontview of a directional backlight apparatus, similar to that of FIG. 47but with the following modifications in which the sensors 208, 214 areremoved and replaced by using the illuminator elements of the array 15in a sensing mode as will be described. Thus in a calibration mode ofoperation, an illuminator element is illuminated and all of the otherilluminator elements are arranged to sense light rather than emit light.The sensed light on the input end is used in the same was as describedwith reference to FIG. 46.

In using the illuminator elements 15 n to sense light, an integratedintensity measurement is made to provide an averaging of total detectedintensity. Thus, while the illuminator elements individually may notprovide high quality measurement, the signal to noise ratio of the arraymay improve the performance. Once a calibration of performance has beenmade and compared to a factory setting, then the required luminousintensity distribution can be achieved as described with reference toFIG. 46. Advantageously the cost of the sensor may be reduced oreliminated, and the sensing may take place over a wide range ofpositions at the input end, providing an averaging of opticalperformance.

FIG. 48 is a schematic diagram illustrating an apparatus to drive anilluminator element for a calibration mode of operation. In this case,the illuminator elements are LEDs 248 and the sensing of light isperformed by operating the LEDs 248 under reverse bias. FIG. 48illustrates a semiconductor p-n junction device including LED 248 thatmay be operated as an illuminator element with a first forward bias andmay also be operated with a reverse bias as a photo-detector. Inoperation with forward bias, LED 248 is driven from drive amplifier 244,which has a signal input 243 and an enable input 251 such that whenswitch 253 is in the GND position 254, amplifier 244 is enabled and theLED 248 emits light 252 in response to signal input 243.

When switch 253 is in position 256, a positive voltage is applied to thecathode of LED 248 such that it is arranged with reversed bias. Inreversed bias LED 248 operates to detect light 250 in cooperation withphoto amplifier 246. Therefore the circuitry 240 can operate p-njunction device 248 as either a LED or a photodetector.

Advantageously this enables the same LED array 15 to function as a photodetector array with suitable circuitry 240. Each source of the array mayhave the arrangement of FIG. 48 and each individual p-n junction devicein the array 15 may have its own photo amplifier 246.

FIG. 49 is a schematic diagram illustrating an illuminator element arrayin calibration mode of operation. Also as shown in FIG. 49, a p-njunction of the array 15 may be driven with forward bias as an LED bydrive amplifier 244 may be separated from the p-n junctions driven asdetectors by switch 249. Similar switches may be configured at otherpositions in array 15. Advantageously, the current output of more thanone p-n junction device 248 may be summed at the virtual ground input ofphoto amplifier 246. Advantageously the sensitivity of detection may beimproved and the number of photo amplifiers 246 may be reduced.

Illuminator elements may typically include white LEDs, in particularLEDs that include a gallium nitride blue light emitting chip and awavelength converting layer that is typically a phosphor arranged toconvert some of the blue light to yellow light. In combination the blueand yellow light may achieve white light output. In operation, the blueand yellow light emitting elements may change output at different rates,and thus the color temperature of the white light output may change withage. Color variations may provide chromatic variations of the opticalwindows, and thus perceived luminance and chrominance changes in theviewing windows. Such changes may increase display flicker for a movingobserver and achieve non-uniformities across the display area. It wouldbe desirable to compensate the output of the illuminator elements forsuch chromaticity changes.

FIG. 50 is a schematic diagram illustrating a front view of anilluminator element array arranged to achieve color correction.Illuminator element array 400 may be arranged in alignment with theinput end 402 of a waveguide 1. Each illuminator element package 412 ofthe array may include first and second gallium nitride chips 404, 408and respective aligned phosphors 406, 410, with the pair aligned inportrait arrangement compared to the input aperture 402. Individualdrive lines 405, 407 are arranged to provide a desired luminous fluxdistribution across the array 15 of illuminator elements. The phosphorsmay be rare earth macroscopic phosphors or may be quantum dot phosphors.

FIG. 51 is a schematic diagram illustrating a front view of a furtherarray of illuminator elements arranged to achieve color correction. Thepackage may be arranged in landscape orientation compared to the inputend 402, and thus the input end may have reduced height, and higherefficiency may be achieved. In such an arrangement, adjacent opticalwindows in the window plane 106 may have different chromatic appearance;however diffusion by asymmetric diffuser 68 of the optical windows inthe lateral direction may be arranged to reduce the chromatic variationin the window plane. Advantageously this arrangement may achieve higherefficiency and lower cross talk of the waveguide 1 for a given height ofthe reflective end 4 compared to the arrangement of FIG. 50.

FIG. 52 is a schematic diagram illustrating a graph of optical windowchromaticity variations and a method to correct chromaticity variations.Thus on a CIE 1931 x-y chromaticity diagram with spectral locus 420 andwhite point locus 426, a factory setting for chromaticity coordinate(being the average of the two sources 404, 406 and 408, 410) may beprovided as point 422. After ageing, the average chromaticity may movein direction 430 to point 424. Such a chromaticity may be measured bymeans of sensor and filter elements 204, 206 and 200, 202 respectivelyas shown in FIG. 46. The control system of FIG. 46 can further providedifferent corrected drive signals along drive lines 405, 407 for eachilluminator element to correct for said variation in output chromaticityand move chromaticity coordinate in direction 428 back to point 422.

Further, the chromaticity and output of illuminator elements may varywith temperature. Thus illuminator elements in a heavily used part ofthe array 15 such as for on-axis positions may operate at a highertemperature than less frequently used parts of the array. Thus, inoperation, the luminance of the illuminator elements may vary with timedue to temperature effects. The sensors 208, 214 may be arranged tooperate during display operation to compensate for said temperaturevariations by means of control system as described above. Thusnon-operating illuminator elements or separate sensors may be arrangedto continuously monitor output luminance and provide adjustment of theilluminator element output dynamically.

FIG. 53 is a schematic diagram illustrating a front view of a array ofilluminator elements and FIG. 54 is a schematic diagram illustrating afront view of a array of illuminator elements and method to correct anilluminator element failure. In operation, viewing window 26 may includelight from adjacent optical windows 249, such that for a given viewingposition in the window plane, the viewing window for example includeslight from at least two and preferably three or more illuminatorelements as shown by region 560. Failure of illuminator elements mayresult in a dip in viewing window profile and can be detected by thesensing system, such as that described in FIG. 46. Such a failure can becompensated as described above, and further by increasing the drive todrive lines 566, 568 to neighbouring illuminator elements, removingdrive to drive line 564.

Pitch 421 of the light source 420 of the array 15 of illuminatorelements 15 n may comprise widths 422, 426 of light with a higher yellowlight content due to predominantly phosphor emission compared to width424 comprising GaN chip area with a higher blue light content. Furthergap 428 (that may comprise parts of the light source mechanical, thermaland electrical package) may have no light emission. The scaled luminousflux is therefore a measure of the average luminous flux across thepitch 421. The pitch 421 may vary across the lateral width of the array15 of illuminator elements 15 n.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1-25. (canceled)
 26. A directional illumination apparatus comprising: awaveguide having an input end; an array of light sources disposed atdifferent input positions in a lateral direction across the input end ofthe waveguide, the waveguide further comprising first and second,opposed guide surfaces for guiding light along the waveguide, thewaveguide being arranged to direct input light from the light sources asoutput light through the first guide surface into optical windows inoutput directions distributed in a lateral direction to a normal to thefirst guide surface that are dependent on the input positions; and acontrol system arranged to selectively operate the light sources todirect light into varying optical windows, and to control the lightsources with respective drive signals to output light with asubstantially suitable luminous flux distribution, the respective drivesignals being configured to compensate for an undesired variation inluminous flux per unit lateral distance across the array of lightsources.
 27. The directional illumination apparatus according to claim26, wherein the control system is arranged to control the light sourceswith the respective drive signals to output light with saidsubstantially suitable luminous flux distribution with the inputpositions across the input end that has a global maximum and reduces oneither side of the global maximum.
 28. The directional illuminationapparatus according to claim 27, wherein the global maximum of thesubstantially suitable luminous flux distribution is in respect of thelight sources aligned with the optical axis of the waveguide.
 29. Thedirectional illumination apparatus according to claim 26, wherein thecontrol system is arranged to control the light sources with therespective drive signals to output light with said substantiallysuitable luminous flux distribution with the input positions thatproduces a luminous intensity of the output light that varies with anangle of the output directions in a luminous intensity distribution thatis Lambertian.
 30. The directional illumination apparatus according toclaim 26, wherein the control system is arranged to control the lightsources with the respective drive signals to output light with saidsubstantially suitable luminous flux distribution with the inputpositions that produces a luminous intensity of the output light thatvaries with an angle of the output directions in an actual luminousintensity distribution having a maximum luminous intensity correspondingto the global maximum of the luminous flux distribution that is greaterthan a notional luminous intensity distribution that is Lambertian andhas the same total luminous intensity over all output directions as theactual luminous intensity distribution. 31-43. (canceled)
 44. Thedirectional illumination apparatus according to claim 26, wherein thefirst guide surface is arranged to guide light by total internalreflection and the second guide surface comprises a plurality of lightextraction features oriented to reflect light guided through thewaveguide in directions allowing exit through the first guide surface asthe output light and intermediate regions between the light extractionfeatures that are arranged to direct light through the waveguide withoutextracting it.
 45. The directional illumination apparatus according toclaim 44, wherein the second guide surface has a stepped shapecomprising facets, that are said light extraction features, and theintermediate regions.
 46. The directional illumination apparatusaccording to claim 45, wherein said facets of the second guide surfaceare arranged to reflect light by total internal reflection, whereby theluminous flux of the output light reflected by the facets, as aproportion of the luminous flux of the input light, varies for differentlight sources, the control system is arranged to control the lightsources with the respective drive signals to output light with saidsubstantially suitable luminous flux distribution across the array oflight sources in a manner that compensates for said variation in theluminous flux of the output light reflected by the facets.
 47. Thedirectional illumination apparatus according to claim 26, wherein thefirst guide surface is arranged to guide light by total internalreflection and the second guide surface is substantially planar andinclined at an angle to reflect light in directions that break the totalinternal reflection for outputting light through the first guidesurface, and the directional illumination apparatus further comprises adeflection element extending across the first guide surface of thewaveguide for deflecting light towards the normal to the lateraldirection.
 48. The directional illumination apparatus according to claim26, wherein the waveguide further comprises a reflective end facing theinput end for reflecting light from the input light back through thewaveguide, the waveguide being arranged to direct input light from thelight sources as output light through the first guide surface afterreflection from the reflective end.
 49. The directional illuminationapparatus according to claim 48, wherein the reflective end has positiveoptical power in a lateral direction across the waveguide.
 50. Thedirectional illumination apparatus according to claim 26, wherein thearray of light sources disposed at different input positions with aconstant pitch in the lateral direction across the input end of thewaveguide, whereby substantially suitable luminous flux distribution isthe actual luminous flux distribution of the light sources. 51-77.(canceled)